**(leetcode_backtracking) Combinations

Combinations

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Question Solution 

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],]

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class Solution {    vector<bool> flag;    vector<vector<int> > ret;    void dfs(int n, int k, int index, int count){        if(k==count){            vector<int> ans;            for(int i=1;i<=n;i++)                if(flag[i])                    ans.push_back(i);            ret.push_back(ans);            return;        }        if(index>=n)  //注意判断，不然越界            return;        flag[index+1] = true;        dfs(n, k, index+1, count+1);        flag[index+1] = false;        dfs(n, k, index+1, count);    }public:    vector<vector<int> > combine(int n, int k) {        for(int i=0;i<=n;i++)            flag.push_back(false);        dfs(n,k,0,0);        return ret;    }};