**(leetcode_backtracking)Permutations

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本文介绍了一个算法解决方案，用于生成给定数字集合的所有可能排列。通过递归回溯的方法，确保了即使在输入包含重复数字的情况下也能得到正确的结果。

Permutations

Total Accepted: 41857 Total Submissions: 131799 My Submissions

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;
            }
        }
    }
};