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]);
}
}
}
};