题目来源 https://leetcode.com/problems/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]. 题意分析 Input: a lis…