DP

第二题时间复杂度要注意了
第三道解码方式简直傻逼

Author: h2pl

src/BFS/原点到特定点的最短路径.java

@@ -7,7 +7,50 @@
  * Created by 周杰伦 on 2018/3/31.
  */
 public class 原点到特定点的最短路径 {
-    public int minPathLength(int[][] grids, int tr, int tc) {
+    //查询最短路径时，节点需要保存此时的长度信息，否则没办法得知长度。
+    public static void main(String[] args) {
+        int [][]a = {{1,1,0,1},
+                     {1,0,1,0},
+                     {1,1,1,1},
+                     {1,0,1,1}};
+        System.out.println(minPathLength(a,3,3));
+    }
+//    public static int minPathLength(int[][] grids, int tr, int tc) {
+//        if (grids == null)return 0;
+//        int m = grids.length;
+//        int n = grids[0].length;
+//        if (tr > m || tr < 0 || tc > n || tc < 0)return 0;
+//        int [][]go = {{0,1},{0,-1},{1,0},{-1,0}};
+//        Queue<int []> queue = new LinkedList<>();
+//        int []start = {0,0};
+//        queue.offer(start);
+//        int len = 0;
+//        while (!queue.isEmpty()) {
+//            int []a = queue.poll();
+//            int r = a[0];
+//            int c = a[1];
+//
+//            for (int i = 0;i < go.length;i ++) {
+//                int []pos = new int[2];
+//                pos[0] = r + go[i][0];
+//                pos[1] = c + go[i][1];
+//                if (pos[0] >= 0 && pos[0] < m && pos[1] >= 0 && pos[1] < n) {
+//                    if (grids[pos[0]][pos[1]] == 1) {
+//                        grids[pos[0]][pos[1]] = 0;
+//                        if (pos[0] == tr && pos[1] == tc)return len;
+//                        queue.offer(pos);
+//                    }
+//
+//                }
+//            }
+//        }
+//        return len;
+//    }
+
+
+
+
+    public static int minPathLength(int[][] grids, int tr, int tc) {
         int[][] next = {{1, 0}, {-1, 0}, {0, 1}, {0, -1}};
         int m = grids.length, n = grids[0].length;
         Queue<Position> queue = new LinkedList<>();
@@ -26,7 +69,7 @@ public int minPathLength(int[][] grids, int tr, int tc) {
         return -1;
     }
 
-    private class Position {
+    private static class Position {
         int r, c, length;
         public Position(int r, int c, int length) {
             this.r = r;

src/DP/分割整数构成字母字符串.java

@@ -0,0 +1,84 @@
+package DP;
+
+/**
+ * Created by 周杰伦 on 2018/4/3.
+ */
+public class 分割整数构成字母字符串 {
+    public static void main(String[] args) {
+        String a = "301";
+        System.out.println(numDecodings(a));
+    }
+
+    public static int numDecodings(String s) {
+        if(s == null || s.length() == 0) {
+            return 0;
+        }
+        int n = s.length();
+        int[] dp = new int[n+1];
+        dp[0] = 1;
+        dp[1] = s.charAt(0) != '0' ? 1 : 0;
+        for(int i = 2; i <= n; i++) {
+            int first = Integer.valueOf(s.substring(i-1, i));
+            int second = Integer.valueOf(s.substring(i-2, i));
+            if(first >= 1 && first <= 9) {
+                dp[i] += dp[i-1];
+            }
+            if(second >= 10 && second <= 26) {
+                dp[i] += dp[i-2];
+            }
+        }
+        return dp[n];
+    }
+
+
+//    public static int numDecodings(String s) {
+//        if (s == null || s.equals(""))return 0;
+//        if (s.equals("0"))return 0;
+//        if (s.length() >= 2) {
+//            String s1 = s.substring(0,2);
+//            int l = Integer.parseInt(s1.substring(0,1));
+//            int r = Integer.parseInt(s1.substring(1,2));
+//            if (l == 0)return 0;
+//            int m = l*10 + r;
+//            if (r == 0 && s.length() == 2 && l < 3) return 1;
+//            if (r == 0 && m > 26)return 0;
+//            if (m > 0 && m <= 26) {
+//                return 2;
+//            }
+//            else return 1;
+//        }
+//        //s = 1234
+//        //dp[1] = 1 dp[2] = 2 dp[3] = 3 dp[4] = 3
+//        int []dp = new int[s.length() + 1];
+//        if (s.charAt(0) == '0') {
+//            return 0;
+//        }
+//        else {
+//            dp[1] = 1;
+//        }
+//        for (int i = 2;i <=s.length();i ++) {
+//            int a = Integer.parseInt(s.substring(i-2,i-1));
+//            int b = Integer.parseInt(s.substring(i - 1,i));
+//            if (a == 0 && b == 0)return 0;
+//            if (b == 0 && i == s.length()) {
+//                if (a * 10 + b > 26) {
+//                    return 0;
+//                }
+//                dp[i] = dp[i - 1] - 1;
+//                break;
+//            }
+//            if (a == 0 || b == 0) {
+//                dp[i] = dp[i - 1];
+//                continue;
+//            }
+//            int num = a * 10 + b ;
+//            if (num > 0 && num <= 26) {
+//                dp[i] = dp[i - 1] + 1;
+//            }else {
+//                dp[i] = dp[i - 1];
+//            }
+//        }
+//        return dp[s.length()];
+//    }
+
+}

src/DP/分割整数的最大乘积.java

@@ -0,0 +1,25 @@
+package DP;
+
+/**
+ * Created by 周杰伦 on 2018/4/3.
+ */
+public class 分割整数的最大乘积 {
+    //2 = 1+1 1 /3 2+1 2/4 2+2 4/5 2+3 6/6 3+3 9
+    // /7 3 2 2 12/8 = 3+3+2 18/9 = 3+3+3 27/10 = 3+3+4 36
+    //dp[0] = 0 dp[1] = 0 dp[2] = 1; dp[3] = dp[2] + i - 1
+    //dp[i] = dp[i-1]
+    public int integerBreak(int n) {
+        int []dp = new int[n + 1];
+        dp[1] = 0;
+        //dp[i]代表以以i为结尾的最大乘积。
+        for (int i = 2;i <=n;i ++) {
+            for (int j = 1;j < i;j ++) {
+                //这一部是将n分成i和j两个数，要么取j要么取j分成的数。
+                //要么取i-j要么取i-j拆分成的数，谁大就取谁。
+                //再将这两部分相乘。并且dp是从左往右算的，可以保证有序。
+                dp[i] = Math.max(dp[i],Math.max(j,dp[j])*Math.max(i - j,dp[i - j]));
+            }
+        }
+        return dp[n];
+    }
+}

src/DP/按平方数来分割整数.java

@@ -0,0 +1,44 @@
+package DP;
+
+/**
+ * Created by 周杰伦 on 2018/4/3.
+ */
+public class 按平方数来分割整数 {
+    public static void main(String[] args) {
+        System.out.println(numSquares(12));
+        System.out.println(numSquares(13));
+
+    }
+    //dp[1] = 1;
+    //dp[2] = 2;
+    //dp[3] = 3;
+    //dp[4] = 1;dp[5] = 2 dp[6] = 3 dp[7]=4 dp[8]=2 dp[9]=3 dp[10]=2 dp[11]=3 dp[12] = 3
+    //dp[i] = dp[i-j] + per
+    //dp[i] = dp[i-1] + 1;huozhe dp[i-j] + which i-j = perfect
+    public static int numSquares(int n) {
+        int []dp = new int[n + 1];
+        dp[1] = 1;
+        dp[0] = 0;
+        for (int k = 2;k <= n;k ++) {
+            dp[k] = k;
+        }
+
+        for (int i = 2;i <= n;i ++) {
+            int j = 1;
+            int min = dp[i];
+            while (i - j*j >= 0 ) {
+                min = Math.min(min,dp[i - j*j] + 1);
+                j ++;
+            }
+            dp[i] = min;
+        }
+
+        return dp[n];
+    }
+    public static boolean isPerfect(int n) {
+        if (Math.pow(n,0.5) - Math.ceil(Math.pow(n,0.5)) == 0) {
+            return true;
+        }
+        return false;
+    }
+}