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回溯法 5大题型
1排列 全排列 有重复数字的排列 2组合 重复元素组合 3求和 重复元素求和 4子集 重复元素子集 5扩展题型 如ip划分 回文数划分 数字打出字母等。 重点在于：递归前标记元素，每次完整递归结束需要清除标记元素。 判断递归结束条件。 细节问题如list的添加，contains使用等。
1 parent 343a3b4 commit 6574692

9 files changed

+231
-6
lines changed

‎src/BackTracking/IP地址划分.java

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/**
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* Created by 周杰伦 on 2018/4/1.
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* 1 file committed: 回溯法
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* 5大题型
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* 1排列 全排列 有重复数字的排列
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* 2组合 重复元素组合
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* 3求和 重复元素求和
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* 4子集 重复元素子集
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* 5扩展题型 如ip划分 回文数划分 数字打出字母等。
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* 重点在于：递归前标记元素，每次完整递归结束需要清除标记元素。
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* 判断递归结束条件。
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* 细节问题如list的添加，contains使用等
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*/
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//1111
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//222 222 222 222

‎src/BackTracking/N皇后.java

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package BackTracking;
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/**
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* Created by 周杰伦 on 2018/4/2.
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*/
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public class N皇后 {
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}

‎src/BackTracking/和为某数的所有组合.java

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package BackTracking;
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import java.util.ArrayList;
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import java.util.Arrays;
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import java.util.Collection;
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import java.util.List;
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/**
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* Created by 周杰伦 on 2018/4/2.
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*/
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public class 和为某数的所有组合 {
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public static void main(String[] args) {
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int []a = {2,3,6,7};
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System.out.println(Arrays.toString((combinationSum(a,7).toArray())));
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}
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public static List<List<Integer>> combinationSum(int[] candidates, int target) {
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List<Integer> arr = new ArrayList<>();
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List<List<Integer>> list = new ArrayList<>();
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backTracking(arr, 0, 0, candidates, target, list);
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return list;
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}
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public static void backTracking(List<Integer> arr,int cur,int sum,int[]nums, int target, List<List<Integer>> list) {
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if (sum == target) {
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list.add(new ArrayList(arr));
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return;
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}
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for (int i = cur;i < nums.length;i ++) {
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//终止条件很重要，否则会死循环
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if (sum + nums[i] <= target) {
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arr.add(nums[i]);
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sum += nums[i];
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backTracking(arr, i, sum, nums, target, list);
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sum -= nums[i];
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arr.remove(arr.size() - 1);
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}
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}
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return;
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}
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}

‎src/BackTracking/和为某数的所有组合2.java

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package BackTracking;
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import java.util.*;
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/**
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* Created by 周杰伦 on 2018/4/2.
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*/
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public class 和为某数的所有组合2 {
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public static void main(String[] args) {
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int []a = {10, 1, 2, 7, 6, 1, 5};
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System.out.println(Arrays.toString((combinationSum2(a,8).toArray())));
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}
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public static List<List<Integer>> combinationSum2(int[] candidates, int target) {
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List<Integer> arr = new ArrayList<>();
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List<List<Integer>> list = new ArrayList<>();
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Arrays.sort(candidates);
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backTracking(arr, 0, 0, candidates, target, list);
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return list;
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}
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public static void backTracking(List<Integer> arr,int cur,int sum,int[]nums, int target, List<List<Integer>> list) {
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if (sum == target) {
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if (!list.contains(arr))
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list.add(new ArrayList(arr));
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return;
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}
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for (int i = cur;i < nums.length;i ++) {
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//终止条件很重要，否则会死循环
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if (sum + nums[i] <= target) {
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arr.add(nums[i]);
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sum += nums[i];
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backTracking(arr, i + 1, sum, nums, target, list);
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sum -= nums[i];
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arr.remove(arr.size() - 1);
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}
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}
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return;
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}
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}

‎src/BackTracking/数字的所有不同组合.java

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package BackTracking;
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import java.util.ArrayList;
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import java.util.Arrays;
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import java.util.List;
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/**
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* Created by 周杰伦 on 2018/4/2.
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*/
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public class 数字的所有不同组合 {
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public static void main(String[] args) {
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int n = 4,k = 2;
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System.out.println(Arrays.toString(combine(n,k).toArray()));
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}
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public static List<List<Integer>> combine(int n, int k) {
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List<Integer> arr = new ArrayList<>();
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List<List<Integer>> list = new ArrayList<>();
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backTracking(arr,1, n, k, list);
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return list;
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}
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public static void backTracking(List<Integer> arr,int left, int n, int k, List<List<Integer>> list) {
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if (arr.size() == k) {
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list.add(new ArrayList(arr));
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return;
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}
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for (int i = left;i <= n;i ++) {
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arr.add(i);
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backTracking(arr, i + 1, n, k, list);
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arr.remove(arr.size() - 1);
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}
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return;
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}
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}

‎src/BackTracking/数独.java

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package BackTracking;
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/**
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* Created by 周杰伦 on 2018/4/2.
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*/
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public class 数独 {
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}

‎src/BackTracking/数组的所有子集.java

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package BackTracking;
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import java.util.ArrayList;
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import java.util.Arrays;
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import java.util.List;
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/**
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* Created by 周杰伦 on 2018/4/2.
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*/
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public class 数组的所有子集 {
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public static void main(String[] args) {
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int []a = {1,2,3};
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System.out.println(Arrays.toString(subsets(a).toArray()));
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}
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public static List<List<Integer>> subsets(int[] nums) {
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int m = nums.length;
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Arrays.sort(nums);
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List<List<Integer>> list = new ArrayList<>();
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ArrayList<Integer> arr = new ArrayList<>();
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for (int i = 1 ;i <= nums.length;i ++) {
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backTracking(arr, nums,0, i, list);
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}
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list.add(new ArrayList<>());
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return list;
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}
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public static void backTracking(ArrayList<Integer> arr, int []nums, int left , int len, List<List<Integer>> list) {
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if (arr.size() == len) {
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list.add(new ArrayList<>(arr));
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return;
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}
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for (int i = left;i < nums.length && i < nums.length;i ++) {
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arr.add(nums[i]);
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backTracking(arr, nums, i + 1,len,list);
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arr.remove(arr.size()-1);
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}
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return;
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}
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}

‎src/BackTracking/数组的所有子集2.java

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package BackTracking;
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import java.util.ArrayList;
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import java.util.Arrays;
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import java.util.List;
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/**
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* Created by 周杰伦 on 2018/4/2.
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*/
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public class 数组的所有子集2 {
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public static void main(String[] args) {
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int []a = {1,2,2};
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System.out.println(Arrays.toString(subsetsWithDup(a).toArray()));
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}
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public static List<List<Integer>> subsetsWithDup(int[] nums) {
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int m = nums.length;
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Arrays.sort(nums);
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List<List<Integer>> list = new ArrayList<>();
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ArrayList<Integer> arr = new ArrayList<>();
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for (int i = 1 ;i <= nums.length;i ++) {
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backTracking(arr, nums,0, i, list);
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}
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list.add(new ArrayList<>());
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return list;
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}
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public static void backTracking(ArrayList<Integer> arr, int []nums, int left , int len, List<List<Integer>> list) {
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if (arr.size() == len) {
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if (!list.contains(arr)) {
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list.add(new ArrayList<>(arr));
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}
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return;
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}
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for (int i = left;i < nums.length && i < nums.length;i ++) {
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arr.add(nums[i]);
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backTracking(arr, nums, i + 1,len,list);
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arr.remove(arr.size()-1);
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}
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return;
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}
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}

‎src/DFS/填充封闭区域.java

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// for (int i = 1;i < m - 1;i ++) {
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// for (int j = 1;j < n - 1;j ++) {
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// if (board[i][j] == 'O' && legal(board, i, j)) {
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// dfs(board,visit,i,j);
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// backTracking(board,visit,i,j);
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// }
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// }
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// }
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// }
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// public boolean dfs(char[][] board, int [][]visit, int x, int y) {
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// public boolean backTracking(char[][] board, int [][]visit, int x, int y) {
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// if (x < 0 || y < 0 || x >= board.length || y >= board[0].length) return false;
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// if (!legal(board, x, y)) return false;
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// if (board[x][y] == 'X') return true;
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// board[x][y] = 'X';
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// return true;
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// }
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// boolean left = dfs(board,visit,x - 1,y);
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// boolean left = backTracking(board,visit,x - 1,y);
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// if (!left) return false;
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// boolean right = dfs(board,visit,x + 1,y);
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// boolean right = backTracking(board,visit,x + 1,y);
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// if (!right) return false;
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// boolean up = dfs(board,visit,x,y + 1);
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// boolean up = backTracking(board,visit,x,y + 1);
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// if (!up) return false;
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// boolean down = dfs(board,visit,x,y - 1);
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// boolean down = backTracking(board,visit,x,y - 1);
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// if (!down) return false;
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//
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// board[x][y] = 'X';

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