LeetCode-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]

.
Solution:
Code:

<span style="font-size:14px;">class Solution {
public:
bool valid(vector<int> &num, int begin, int end) {
for (int i = begin; i < end; ++i)
if (num[i] == num[end]) return false;
return true;
}

void helper(vector<vector<int> > &results, vector<int> &num, const int &length, int index) {
if (index == length) {
results.push_back(num);
return;
}
helper(results, num, length, index+1);
for (int i = index+1; i < length; ++i)
if (valid(num, index, i)) {
swap(num[index], num[i]);
helper(results, num, length, index+1);
swap(num[index], num[i]);
}
}

vector<vector<int> > permuteUnique(vector<int> &num) {
vector<vector<int> > results;
const int length = num.size();
if (length == 0) return results;
if (length == 1) {
results.push_back(num);
return results;
}
sort(num.begin(), num.end());
helper(results, num, length, 0);
return results;
}
};</span>