77. Combinations(回溯)

Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.

Example:

Input: n = 4, k = 2
Output:
[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

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

原文地址：https://www.cnblogs.com/zle1992/p/8902020.html