leetcode[47]Permutations II

Given a collection of numbers that might contain duplicates, return all possible unique permutations.

For example,
[1,1,2] have the following unique permutations:
[1,1,2], [1,2,1], and [2,1,1].

class Solution {
public:
bool isSwap(vector<int> &num, int i, int j)
{
    int ii=i;
    while(num[ii]!=num[j]&&ii<j)ii++;
    if (ii==j)return true;
    else return false;
}
void perm(vector<int> &num, int i, int N, vector<vector<int>> &res)
{
    if(i==N)
        res.push_back(num);
    for (int j=i;j<N;j++)
    {
        if (!isSwap(num,i,j))continue;
        swap(num[i],num[j]);
        perm(num,i+1,N,res);
        swap(num[j],num[i]);
    }
}
vector<vector<int> > permuteUnique(vector<int> &num) 
{
    vector<vector<int> > res;
    sort(num.begin(),num.end());
    perm(num,0,num.size(),res);
    return res;
}
};