LeetCode题解–46. Permutations

链接

LeetCode题目：https://leetcode.com/problems/permutations

难度：Medium

题目

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

生成n个数字的所有排列情况。

分析

为了生成n个数字的所有排列情况，可以先生成后面n-1个数字的所有排列情况，然后第1个数字插到任意位置。而生成n-1个数字的排列也可以先从生成n-2个数字开始，以此类推，最后生成1个数字的排列情况只需要返回该数字。

代码

class Solution {
public:
    vector<vector<int>> permute(vector<int> &nums) {
        vector<vector<int>> res;
        for (int i = 0; i < nums.size(); i++) {
            vector<vector<int>> temp = permute2(nums, 0, i);
            for (auto x:temp) {
                res.push_back(x);
            }
        }
        return res;
    }

    vector<vector<int>> permute2(vector<int> &nums, int old_pos, int new_pos) {
        vector<vector<int>> res;
        if (nums.size() == 1) {
            res.push_back(nums);
        } else {
            vector<int> temp;
            for (int i = 0; i < old_pos; i++) {
                temp.push_back(nums[i]);
            }
            for (int i = old_pos + 1; i < nums.size(); i++) {
                temp.push_back(nums[i]);
            }
            auto all_permutes = permute(temp);
            for (auto x:all_permutes) {
                vector<int> temp2;
                int i, j;
                for (i = 0, j = 0; j < new_pos; i++, j++) {
                    temp2.push_back(x[i]);
                }
                temp2.push_back(nums[old_pos]);
                for (; j < x.size(); i++, j++) {
                    temp2.push_back(x[i]);
                }
                res.push_back(temp2);
            }
        }
        return res;
    }

};