46. 全排列

给定一个没有重复数字的序列，返回其所有可能的全排列。

示例:

输入: [1,2,3]
输出:
[
  [1,2,3],
  [1,3,2],
  [2,1,3],
  [2,3,1],
  [3,1,2],
  [3,2,1]
]

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/permutations
著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

解法一：

class Solution {
public:
    vector<vector<int>> permute(vector<int>& nums) 
    {
        vector<vector<int>> res;
        solution(nums, 0, res);
        return res;      
    }
    void solution(vector<int> &nums, int start, vector<vector<int>> &res)
    {
        if(start == nums.size())
        {
            res.push_back(nums);
            return;
        }
        for(int i = start; i < nums.size(); ++i)
        {
            swap(nums[i], nums[start]);
            solution(nums, start+1, res);
            swap(nums[i], nums[start]);
        }
    }
};

解题思路二：

class Solution {
private:
    vector<vector<int>> rems;
public:
    vector<vector<int>> permute(vector<int>& nums) 
    {
        vector<int> out;
        vector<int> flag(nums.size(), 0);
        backtracking(nums, 0, out, flag);
        return rems;  
    }

    void backtracking(vector<int> &nums, int start, vector<int> &out, vector<int> &flag)
    {
        if(start == nums.size())
        {
            rems.push_back(out);
            return;
        }
        for(int i = 0; i < nums.size(); ++i)
            if(flag[i] == 0)
            {
                out.push_back(nums[i]);
                flag[i] = 1;
                backtracking(nums, start + 1, out, flag);
                out.pop_back();
                flag[i] = 0;
            }
    }
};