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]);
            }
        }
    } 
};