47. Permutations II (Medium)

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],
  [2,1,1]
]

Solution:

class Solution {
    void dfs(vector<int> nums, int i, vector<vector<int>>& res) {
        if (i == nums.size()-1) {res.push_back(nums); return;}
        for (int k = i; k < nums.size(); ++k) {
            if (k != i && nums[k] == nums[i]) continue;
            swap(nums[i], nums[k]);
            dfs(nums, i+1, res);
        }
    }
public:
    vector<vector<int>> permuteUnique(vector<int>& nums) {
        sort(nums.begin(), nums.end());
        vector<vector<int>> res;
        dfs(nums, 0, res);
        return res;
    }
};