30 Day Challenge Day 14 | Leetcode 77. Combinations

题解

Medium | Backtracking

经典的排列组合题，使用回溯法。必须熟练。

class Solution {
public:
    vector<vector<int>> combine(int n, int k) {
        vector<vector<int>> combinations;
        vector<int> combination;
        helper(n, k, 1, combination, combinations);
        return combinations;
    }
    
    void helper(int n, int k, int pos, vector<int>& combination, vector<vector<int>>& combinations) {
        if(k == 0) {
            combinations.push_back(combination);
            return;
        }
        
        for(int i = pos; i <= n; i++) {
            combination.push_back(i);
            helper(n, k-1, i+1, combination, combinations);
            combination.pop_back();
        }
    }
};