[leetcode] Permutations

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From : https://leetcode.com/problems/permutations/

Given a collection of 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].

class Solution {public:    vector<vector<int>> permute(vector<int>& nums) {        vector<vector<int>> ans;        vector<int> cur;        vector<bool> notUse(nums.size(), true);        insert(ans, cur, nums, notUse, 0);        return ans;    }    void insert(vector<vector<int>>& ans, vector<int>& cur, vector<int>& nums, vector<bool>& notUse, int used) {        if(used == nums.size()) {ans.push_back(cur); return; }        for(int i=0, len=nums.size(); i<len; i++) {            if(notUse[i]) {                notUse[i] = false;                cur.push_back(nums[i]);                insert(ans, cur, nums, notUse, used+1);                cur.pop_back();                notUse[i] = true;            }        }    }};

改进：

class Solution {public:    vector<vector<int> > permute(vector<int> &num) {        vector<vector<int> > ans;dfs(ans, num, 0);return ans;    }void dfs(vector<vector<int> >& ans, vector<int>& num, int cur) {if(num.size() == cur) { ans.push_back(num); return; }for(int i = cur; i < num.size(); ++i) {swap(num[cur], num[i]);dfs(ans, num, cur+1);swap(num[cur], num[i]);}}};

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