题目描述:

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:

1
2
3
4
5
6
7
8
[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

从[1,n]中选出k个数, 这道题只要求组合, 不要求排列, 因此用递归可以比较容易的解决.

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
class Solution {
    vector<vector<int>> kElement;
public:
    vector<vector<int>> combine(int n, int k) {
        vector<int> v;
        getKElement(n, k, 1, v);
        return kElement;
    }
    
    void getKElement(int n, int k, int start, vector<int> &v){
        if(k == 1){
            for(int i = start; i <= n; i++){
                v.push_back(i);
                kElement.push_back(v);
                v.pop_back();
            }
            return;
        }
        for(int i = start; i <= n - k + 1; i++){
            v.push_back(i);
            getKElement(n, k - 1, i + 1, v);
            v.pop_back();
        }
    }
};