Leetcode: 77. Combinations

Description

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

Example

If n = 4 and k = 2, a solution is:

[
    [2,4],
    [3,4],
    [2,3],
    [1,2],
    [1,3],
    [1,4],
]

思路

• 常规的回溯

代码

class Solution {
public:
    vector<vector<int>> combine(int n, int k) {
        vector<vector<int>> res;
        vector<int> vec;
        getResult(res, vec, n, 1, k);
        return res;
    }
    
    void getResult(vector<vector<int>>& res, 
        vector<int> &vec, int n, int t, int k){
        
        if(vec.size() == k){
            res.push_back(vec);
            return;
        }    
        
        for(int i = t; i <= n; ++i){
            vec.push_back(i);
            getResult(res, vec, n, i + 1, k);
            vec.pop_back();
        }
        
    }
};