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],  [3,2,1]]

刚开始天真地以为用迭代法可以做出来，可是真正做的时候发现很麻烦，其是这道题又是一道典型的回溯法，可以维护一个visited数组，用来记录哪些数字是已经被访问过的。

class Solution {public:    vector<vector<int>> permute(vector<int>& nums) {        vector<vector<int>> ans;        vector<int> out;        vector<int> visited(nums.size(), 0);        permuteDFS(nums, 0, visited, out, ans);        return ans;    }    void permuteDFS(vector<int>& nums, int level, vector<int>& visited, vector<int>& out, vector<vector<int>>& ans) {        if (level == nums.size()) ans.push_back(out);        else {            for (int i = 0; i < nums.size(); i++) {                if (visited[i] == 0) {                    visited[i] = 1;                    out.push_back(nums[i]);                    permuteDFS(nums, level + 1, visited, out, ans);                    out.pop_back();                    visited[i] = 0;                }            }        }    }  };