【leetcode】【46】Permutations

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一、问题描述

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].

二、问题分析

同样是backtracking，与combinations极其类似，不过该题中需要用到一个visited数组来标记一下某个元素是否访问过。

三、Java AC 代码

public List<List<Integer>> permute(int[] nums) {List<List<Integer>> list = new ArrayList<List<Integer>>();List<Integer> item = new ArrayList<Integer>();if (nums==null || nums.length==0) {return list;}boolean[] visited = new boolean[nums.length];dfsHelper(list, item, nums, visited);return list;}public void dfsHelper(List<List<Integer>> list, List<Integer> item, int[] nums, boolean[] visited){if (item.size()==nums.length) {list.add(new ArrayList<Integer>(item));return ;}for (int i = 0; i < nums.length; i++) {if (!visited[i]) {item.add(nums[i]);visited[i] = true;dfsHelper(list, item, nums, visited);visited[i] = false;item.remove(item.size()-1);}}}

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