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].

解题代码如下：

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class Solution {
public:
	vector
  
   > result;
public:
	vector
   
     > permute(vector
    
      &num) { int length=num.size(); perm(num,0,length-1); return result; } void perm(vector
     
       &num,int k,int m){ if(k==m) result.push_back(num); for(int i=k;i<=m;i++){ swap(num[k],num[i]); perm(num,k+1,m); swap(num[k],num[i]); } } void swap(int &a,int &b){ int temp=a; a=b; b=temp; } };
     
    
   
  

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