LeetCode 77. Combinations

77. Combinations
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],
]

题目大意：给两个整数n和k，返回所有的k个数字组合，这些数字只能从1...n中选择～
分析：从cur == 0，cnt == 0开始，每次将cur + 1 ～ n之间的数字放入row中，并将cnt + 1，然后继续深度优先搜索直到cnt == k为止将row放入result中作为结果之一，不要忘记dfs遍历后还要将当前元素pop_back()出来，最后返回result～

class Solution {public:    vector<vector<int>> combine(int n, int k) {        this->n = n, this->k = k;        dfs(0, 0);        return result;    }private:    int n, k;    vector<vector<int>> result;    vector<int> row;    void dfs(int cur, int cnt) {        if (cnt == k) {            result.push_back(row);            return;        }        for (int i = cur + 1; i <= n; i++) {            row.push_back(i);            dfs(i, cnt + 1);            row.pop_back();        }    }};