Permutations 1

Given an array nums of distinct integers, return all the possible permutations. You can return the answer in any order.

Example:
// Input: nums = [1,2,3]
// Output: [[1,2,3],[1,3,2],[2,1,3],[2,3,1],[3,1,2],[3,2,1]]

void solve(vector<int> &nums, vector<vector<int>> &ans, int i)
        {
            if (i == nums.size())
            {
                ans.push_back(nums);
                return;
            }
            for (int j = i; j < nums.size(); j++)
            {
                swap(nums[i], nums[j]);
                solve(nums, ans, i + 1);
                swap(nums[i], nums[j]);
            }
        }
    vector<vector<int>> permute(vector<int> &nums)
    {
        vector<vector<int>> ans;
        solve(nums, ans, 0);
        return ans;
    }

Permutations 2

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

Example:
// Input: nums = [1,2,3]
// Output: [[1,2,3],[1,3,2],[2,1,3],[2,3,1],[3,1,2],[3,2,1]]

 void solve(vector<int> &nums, vector<vector<int>> &ans, int i)
    {
        if (i == nums.size())
        {
            ans.push_back(nums);
            return;
        }
        unordered_set<int> seen;
        for (int j = i; j < nums.size(); j++)
        {
            if (seen.count(nums[j]))
                continue;
            seen.insert(nums[j]);
            swap(nums[i], nums[j]);
            solve(nums, ans, i + 1);
            swap(nums[i], nums[j]);
        }
    }
    vector<vector<int>> permuteUnique(vector<int> &nums)
    {
        vector<vector<int>> ans;

        solve(nums, ans, 0);
        return ans;
    }