zoukankan      html  css  js  c++  java
  • 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].

    Analysis:

    The idea of this classic problem is to use backtracking.
    We want to get permutations, which is mainly about swap values in the list.
    Consider:
    a --> a
    ab --> ab, ba
    abc --> abc, acb, bac, bca, cab, cba.
    ...
    where for the length of n, the permutations can be generated by
    (1) Swap the 1st element with all the elements, including itself.
           (2) Then the 1st element is fixed, go to the next element.
           (3) Until the last element is fixed. Output.
    It's more clear in the figure above. The key point is to make the big problem into smaller problem, here is how to convert the length n permutation into length n-1 permutation problem.

    public class Solution {
        
        public ArrayList<ArrayList<Integer>> permute(int[] num){
           ArrayList<ArrayList<Integer>> result = new  ArrayList<ArrayList<Integer>>();
            ArrayList<Integer> output = new ArrayList<Integer>();
            boolean[] visited = new boolean[num.length];
           getPermutation(0, num, output, result, visited);
           return result;
        }
        
        private void getPermutation(int depth, int[] num, ArrayList<Integer> output, ArrayList<ArrayList<Integer>> result, boolean[] visited){
            if(depth == num.length){
                 ArrayList<Integer> tmp = new ArrayList<Integer>();
                 tmp.addAll(output);
                 result.add(tmp);
            }
            
            for(int i= 0 ; i< num.length; i++){
                if(visited[i])
                    continue;
                visited[i] = true;
                output.add(num[i]);
                getPermutation(depth+1, num, output, result, visited);
                output.remove(output.size() -1);
                 visited[i] = false;
            }
            
        }
    }
  • 相关阅读:
    Django REST framework的解析器与渲染器
    python基础之 数据格式化
    REST framework 之 分页
    Django REST framework 之 认证 权限 限制
    DjangoRestFrameWork 版本控制
    DjangoRESTFrameWork中的视图
    浏览器跨域问题
    初识REST
    vue之生命周期
    vue组件
  • 原文地址:https://www.cnblogs.com/RazerLu/p/3536039.html
Copyright © 2011-2022 走看看