hdu1518-dfs

Posted 2020-12-18 lijiahui-123

 

import java.util.Arrays;

import java.util.Comparator;

import java.util.Scanner;

 

public class hdu1518DFS{

    private static boolean [] vis;

    private static Integer [] array;

    private static int edge;

    public static void main(String[] args) {

        Scanner in = new Scanner(System.in);

        int cases,n,sum;

        cases = in.nextInt();

        while(cases-->0){

            n = in.nextInt();

            sum = 0;

            vis = new boolean[n];

            array = new Integer[n];

            for(int i = 0; i < array.length; ++i){

                array[i] = in.nextInt();

                sum += array[i];

            }

            edge = sum >>2;//sum/4

            if(sum%4 == 0 && edge >= array[0]){   //剪枝，在剪枝后再进行操作会减少运行时间

                Arrays.sort(array,new Comparator<Integer>(){    //从大到小排序

                    @Override

                    public int compare(Integer o1, Integer o2){

                        return o2-o1;

                    }

                });

                if(dfs(0,0,0)){

                    System.out.println("yes");

                }else{

                    System.out.println("no");

                }

            }else{

                System.out.println("no");

            }

        }

    }

    private static boolean dfs(int curLen, int num, int cur){

        if(num == 3){

            return true;

        }

        if(curLen == edge){

            if(dfs(0,num+1,num+1)){

                return true;

            }else{

                return false;

            }

        }else{ 

            for(int i = cur; i < array.length; ++i){

                if(!vis[i] && curLen+array[i] <= edge){

                     vis[i] = true;

                     if(dfs(curLen+array[i],num,i+1)){  //这里时重点，大幅减小搜索范围

                         return true;

                     }

                     vis[i] = false;

                }

            }

        }

        return false;

    }

}

这里的剪枝很重要