第十五周 leetcode 77. Combinations（Medium）

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

解题思路：
深度优先搜索，k层深度，不够k层就回溯

代码：

class Solution {public:    vector<vector<int> > combine(int n, int k) {        vector<vector<int> > res;        int a[k];        dfs(res,a,0,k,0,n);        return res;    }    void dfs(vector<vector<int> > &res,int a[],int depth,int k,int cur,int n)    {        if(depth == k){            vector<int> temp;            for(int i = 0; i < k; i++)                    temp.push_back(a[i]);            res.push_back(temp);            return ;        }        for(int i = cur;i < n;i++){            if(n - cur < k - depth)                     return ;    //剩下的数凑不齐k个，回溯            a[depth] = i + 1;            dfs(res,a,depth + 1,k,i + 1,n);        }    }};

代码运行结果：