leetcode 45: 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].

 

 

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> > rel;        int level = 0;                permuteRec( rel, num, level);                return rel;    }    private:      void permuteRec( vector<vector<int> > & rel, vector<int> &num, int level) {        if( level == num.size() )  {            rel.push_back( num );            return;        }                for( int i=level; i<num.size();i++){            if( i != level) {                swap(num[i], num[level]);            }            permuteRec( rel, num, level+1);            if( i != level) {                swap( num[i], num[level]);            }        }    } };