[leetcode]Combinations

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Combinations

 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],]

演算法：

思路1： 迭代，k個數就用k層迭代。貌似實現起來也是略有難度的

 

思路2： DFS，不囉嗦

 1 public class Solution { 2     List<List<Integer>> result = new ArrayList<List<Integer>>(); 3     public List<List<Integer>> combine(int n, int k) { 4         if(n < k || k < 0 || n < 0) return result; 5         List<Integer> list = new ArrayList<Integer>(); 6         dfs(list,1, n, k); 7         return result; 8     } 9     private void dfs(List<Integer> list,int start,int n,int left){10         if(left == 0){11             result.add(new ArrayList<Integer>(list));12             return ;13         }14         for(int i = start; i <= n ; i++){15             list.add(i);16             dfs(list, i + 1, n, left - 1);17             list.remove(list.size() - 1);18         }19     }20 }