permutations.cpp

// Time:  O(n * n!)
// Space: O(n)

class Solution {
public:
    /**
     * @param nums: A list of integers.
     * @return: A list of permutations.
     */
    vector<vector<int>> permute(vector<int> nums) {
        if (nums.empty()) {
            return {};
        }
        vector<vector<int>> result;
        vector<bool> used(nums.size(), false);
        vector<int> ans;

        sort(nums.begin(), nums.end());
        permuteHelper(nums, &used, &ans, &result);
        return result;
    }

    void permuteHelper(const vector<int> &A, vector<bool> *used,
                       vector<int> *ans, vector<vector<int>> *result) {
        if (ans->size() == A.size()) {
            result->emplace_back(*ans);
            return;
        }

        for (size_t i = 0; i < A.size(); ++i) {
            if ((*used)[i]) {
                continue;
            }
            (*used)[i] = true;
            ans->emplace_back(A[i]);
            permuteHelper(A, used, ans, result);
            ans->pop_back();
            (*used)[i] = false;
        }
    }
};

// Time:  O(n * n!)
// Space: O(n)
// STL solution.
class Solution2 {
public:
    /**
     * @param nums: A list of integers.
     * @return: A list of permutations.
     */
    vector<vector<int>> permute(vector<int> nums) {
        if (nums.empty()) {
            return {};
        }
        vector<vector<int>> result;
        sort(nums.begin(), nums.end());
        do {
            result.emplace_back(nums);
        } while (next_permutation(nums.begin(), nums.end()));
        return result;
    }
};