//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 }