https://leetcode.com/problems/combinations/description/
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],
]
1 public List<List<Integer>> combine(int n, int k) {
2 List<List<Integer>> res = new ArrayList<>();
3 if (n == 0 || k<0) {
4 return res ;
5 }
6 dfs(n,k,0,new ArrayList<>(), res);
7 return res ;
8 }
9
10 private void dfs(int n, int k, int index , ArrayList<> subRes, List<List<Integer>> res ) {
11 //terminate :
12 if (index == k) {
13 //deep copy
14 res.add(new ArrayList<>(subRes)) ;
15 return res ;
16 }
17 for (int i = index; i <= n ; i++ ) {
18 //add
19 subRes.add(i) ;
20 //不能有重复
21 dfs(n, k, i+1, subRes, res);
22 //remove
23 subRes.remove(subRes.size()-1) ;
24 }
25 }
