**(leetcode_backtracking)Permutations

Permutations

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Question Solution 

Given a collection of numbers, return all possible permutations.

For example,
[1,2,3] have the following permutations:
[1,2,3], [1,3,2], [2,1,3], [2,3,1], [3,1,2], and [3,2,1].

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class Solution {    vector<vector<int> > result;    vector<int> single;    vector<bool> flag;public:    vector<vector<int> > permute(vector<int> &num) {        if(num.size()==0)            return result;                    sort(num.begin(), num.end());         for(int i=0; i<num.size(); i++){            flag.push_back(false);            single.push_back(num[i]);        }        bk(num, 0);        return result;    }    void bk(vector<int> &num, int index){        if(index==num.size()){            result.push_back(single);            return;        }        int preNum = num[0]-1;        for(int i=0; i<num.size(); i++){            if(!flag[i]&&preNum!=num[i]){ //注意会出现重复数字！！                preNum=num[i];                flag[i]=true;                single[index]=num[i];                bk(num, index+1);                flag[i]=false;            }        }    }};