77. Combinations

简单题dfs

class Solution {public:    vector<vector<int>>ve;    vector<int>vee;    void dfs(int n, int k, int now, int num){        vee.push_back(now);        if(num + 1 == k){            ve.push_back(vee);        }        else{            for(int i = now + 1; i <= n; ++ i)                dfs(n, k, i, num + 1);        }        vee.pop_back();    }    vector<vector<int>> combine(int n, int k) {        if(n < k) return ve;        for(int i = 1; i <= n; ++ i){            vee.clear();            dfs(n, k, i, 0);        }        return ve;    }};