LeetCode 46. Permutations

Given a collection of distinct numbers, return all possible permutations.

For example,
[1,2,3] have the following permutations:
[1,2,3], [1,3,2], [2,1,3], [2,3,1],[3,1,2], and [3,2,1].

Classical backtrack.

In order to make the output in sequence, sort the inputs first.

   void permute(vector<int>& nums, int depth, vector<vector<int>>& res, vector<int>& path, vector<int>& used) {        if(depth == nums.size()) res.push_back(path);  // depth variable can be saved, if here checks path.size() == nums.size()        for(int i = 0; i < nums.size(); ++i) {            if(used[i] == false) {                used[i] = true;                        // use a flag to mark that the value has been used.                path.push_back(nums[i]);                permute(nums, depth + 1, res, path, used);                used[i] = false;                path.pop_back();            }        }    }    vector<vector<int>> permute(vector<int>& nums) {        if(nums.size() == 0) return {};        sort(nums.begin(), nums.end());        vector<vector<int>> res;        vector<int> path;        vector<int> used(nums.size(), false);        permute(nums, 0, res, path, used);        return res;    }