[leetcode] permutations

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].

最简单的当属用递归的方法去解：

class Solution {
public:
    vector<vector<int> > permute(vector<int> &num) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        vector<vector<int> >ret;
        int n=num.size();
        _permute(num,n,n-1,ret);
        return ret;
    }
    void _permute(vector<int>& array,int n,int depth,vector<vector<int> >&ret){
        if(depth==0){
            ret.push_back(array);
            return;
        }
        for(int i=0 ; i<=depth ; i++){
            swap(array[i],array[depth]);
            _permute(array,n,depth-1,ret);
            swap(array[i],array[depth]);
        }
        return;        
    }
};