【leetcode刷提笔记】Permutations

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].



用深度搜索的方法搜索下面的一棵树(示例,非完整的树),每次搜索到叶子时经过的路径就是一个排列。

代码如下:

 1 public class Solution {
 2     public List<List<Integer>> permute(int[] num) {
 3         List<List<Integer>> answerList = new ArrayList<List<Integer>>();
 4         ArrayList<Integer> currentList = new ArrayList<Integer>();
 5         boolean[] visited = new boolean[num.length];
 6         DS(answerList, visited, num, currentList);
 7         return answerList;
 8     }
 9
10     public void DS(List<List<Integer>> answerList,boolean[] visited,int[] num,ArrayList<Integer> currentList){
11         boolean find = true;
12         for(int i = 0;i < num.length;i++){
13             if(!visited[i]){
14                 currentList.add(num[i]);
15                 visited[i]= true;
16                 DS(answerList, visited, num, currentList);
17                 visited[i]= false;
18                 currentList.remove(currentList.size()-1);
19                 find = false;
20             }
21         }
22         if(find){
23             ArrayList <Integer> temp = new ArrayList<Integer>(currentList);
24             answerList.add(temp);
25         }
26     }
27 }

上述代码中DS函数为递归深度优先搜索函数,answerList记录最终得到的所有排列,visited数据记录在某个时间点对应节点是否在访问过的路径上,currentList为当前走过的路径。要注意的一点是找到一条新的路径后,要为这条路径申请内存空间存放它(代码第23行所示),否则currentList被修改的时候已经存放到answerList中的排列也会被修改。

【leetcode刷提笔记】Permutations

时间: 2024-11-06 07:21:56

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