basics

Author: idf

src/Combinations/Solution.java

@@ -0,0 +1,57 @@
+package Combinations;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * User: Danyang
+ * Date: 1/24/2015
+ * Time: 11:05
+ *
+ * Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.
+
+ For example,
+ If n = 4 and k = 2, a solution is:
+
+ [
+ [2,4],
+ [3,4],
+ [2,3],
+ [1,2],
+ [1,3],
+ [1,4],
+ ]
+ */
+public class Solution {
+    /**
+     * Notice:
+     * 1. debug recursive
+     * @param n
+     * @param k
+     * @return
+     */
+    public List<List<Integer>> combine(int n, int k) {
+        List<List<Integer>> ret = new ArrayList<>();
+        dfs(n, k, 1, new ArrayList<>(), ret);
+        return ret;
+    }
+
+    void dfs(int n, int k, int next, List<Integer> cur, List<List<Integer>> ret) {
+        if(cur.size()>k)
+            return ;
+        if(cur.size()==k) {
+            ret.add(new ArrayList<>(cur));
+            return ;
+        }
+        for(int i=next; i<=n; i++) {
+            cur.add(i);
+            dfs(n, k, i+1, cur, ret);
+            cur.remove(cur.size()-1);
+        }
+    }
+
+    public static void main(String[] args) {
+        List<List<Integer>> ret = new Solution().combine(4, 2);
+        System.out.println(ret);
+    }
+}