【leetcode】【46】Permutations

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);
			}
		}
	}