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LeetCode Permutations

2024-07-27 19:08:30   209人阅读

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].

 

采用递归的思路，一次加一个数，每次更新上一次的结果。

 1 public class Solution { 2  3     public List<List<Integer>> permute(int[] num) { 4         List<List<Integer>> lastList=new ArrayList<List<Integer>>(); 5         List<List<Integer>> currList=new ArrayList<List<Integer>>(); 6             if (num==null) { 7                 return lastList; 8             } 9             10             List<Integer> eleList=new ArrayList<>();11             eleList.add(num[0]);12             lastList.add(eleList);13             14             if (num.length==1) {15                 return lastList;16             }17             for (int i = 1; i < num.length; i++) {18                 for (List<Integer> l : lastList) {19                     currList.addAll(insertList(num[i], l));20                     21                 }22                 lastList.clear();23                 lastList.addAll(currList);24                 currList.clear();25         }26             return lastList;27             28     }29     30     private List<List<Integer>> insertList(int num,List<Integer> list) {31             List<List<Integer>> retNew=new ArrayList<List<Integer>>();32             33             for (int i = 0; i <= list.size(); i++) {34                     List<Integer> currlist=new ArrayList<>();35                 currlist.addAll(list);36                 currlist.add(i, num);37                 retNew.add(currlist);38                 currlist=null;39             }40             return retNew;41         42     }43 }

 

LeetCode Permutations


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