47. Permutations II

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

Example:

Input: [1,1,2]
Output:
[
  [1,1,2],
  [1,2,1],
  [2,1,1]
]

AC code:

class Solution {
public:
    vector<vector<int>> permuteUnique(vector<int>& nums) {
        vector<vector<int>> v;
        sort(nums.begin(), nums.end());
        recursion(nums, v, 0, nums.size()-1);
        return v;
    }

    void recursion(vector<int> nums, vector<vector<int>>& v, int i, int j) {
        if (i == j) {
            v.push_back(nums);
            return;
        }
        for (int k = i; k <= j; ++k) {
            if (i != k && nums[i] == nums[k])
                continue;
            swap(nums[i], nums[k]);
            recursion(nums, v, i+1, j);
        }
    }
};

Runtime: 28 ms, faster than 46.26% of C++ online submissions for Permutations II.