[LeetCode] Combinations

//递归结发，计算正确，但是会timeout
 1 public class Solution {
 2     public List<List<Integer>> combine(int n, int k) {
 3         int [] num = new int[n+1];
 4         for (int i=0; i<=n; i++) num[i] = i;
 5         List<List<Integer>> result = new ArrayList<List<Integer>>();
 6         
 7         generateCombine(result, num, 1, k);
 8         
 9         return result;
10     }
11     
12     public void generateCombine(List<List<Integer>> result, int [] num, int nowIndex, int k) {
13         if (nowIndex>k) {
14             List<Integer> oneResult = new ArrayList<Integer>();
15             for (int i=0; i<=k; i++) oneResult.add(num[i]);
16             result.add(oneResult);
17         } else {
18             for (int i=nowIndex; i<num.length; i++) {
19                 swap(num, nowIndex, i);
20                 generateCombine(result, num, nowIndex+1, k);
21                 swap(num, nowIndex, i);
22             }
23         }
24     }
25     
26     public void swap(int [] num, int i, int j) {
27         int tmp = num[i];
28         num[i] = num[j];
29         num[j] = tmp;
30     }
31 }