DFS

DFS

261. 树状结构查询

import java.util.Scanner;
import java.util.*;

// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
    static Map<String, Set<String>> map = new HashMap<>();  //  使用 set 避免录入重复信息
    static StringBuilder sb = new StringBuilder();

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int row = Integer.parseInt(in.nextLine());
        for (int i = 0; i < row; i++) {
            String[] split = in.nextLine().split(" ");
            String son = split[0];
            String father = split[1];
            Set<String> orDefault = map.getOrDefault(father, new HashSet<>());
            orDefault.add(son);
            map.put(father, orDefault);
        }
        String target = in.nextLine();
        select(target);
        String[] split = sb.toString().trim().split(" ");
        String[] res = Arrays.copyOfRange(split, 1, split.length);  //  截断首个
        Arrays.sort(res);
        for (String re : res) {
            System.out.println(re);
        }
    }

    public static void select(String str){
        sb.append(str + " ");   //  递归查找！！！
        if (!map.containsKey(str)){
            return;
        }
        Set<String> set = map.get(str);
        for (String s : set) {
            select(s);
        }
    }
}

239. 目录删除

import java.util.Scanner;
import java.util.*;

// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
    static Map<Integer, Set<Integer>> map = new HashMap<>();

    public static void main(String[] args) {
         Scanner in = new Scanner(System.in);
        int count = Integer.parseInt(in.nextLine());
        for (int i = 0; i < count; i++) {
            String[] split = in.nextLine().split(" ");
            int son = Integer.parseInt(split[0]);
            int father = Integer.parseInt(split[1]);
            Set<Integer> orDefault = map.getOrDefault(father, new HashSet<>());
            orDefault.add(son);
            map.put(father, orDefault);
        }
        int target = Integer.parseInt(in.nextLine());
        del(target);
        TreeSet<Integer> res = new TreeSet<>();
        for (Set<Integer> value : map.values()) {
            for (Integer integer : value) {
                if (target != integer){
                    res.add(integer);
                }
            }
        }
        StringBuilder sb = new StringBuilder();
        for (Integer re : res) {
            sb.append(re + " ");
        }
        System.out.println(sb.toString().trim());
    }
    
   public static void del(Integer target){
        if (!map.containsKey(target)){ //  如果是叶子节点，没资格做 father（即作为 key），直接返回
            return;
        }
        Set<Integer> set = map.get(target);
        map.remove(target);  //  map.remove() 方法返回的是被删除的 value

        for (Integer integer : set) {
            del(integer);
        }
    }
}

166. 分糖果

import java.util.Scanner;
import java.util.*;


// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        long count = in.nextLong();
        System.out.println(getMin(count));
    }
     public static int getMin(long count){
        if (count == 1){
            return 0;
        }

        if (count % 2 == 0){
            return getMin(count / 2) + 1;  //  平均分配：1 次数
        }

        return Math.min(getMin((count + 1) / 2), getMin((count - 1) / 2) ) + 2; 平均分配 1 次 + 【取出、或者放回的 1 次】
    }
}

解密犯罪时间

import java.util.Scanner;
import java.util.*;
// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
     static StringBuilder sb = new StringBuilder();
    static List<String> list = new ArrayList<>();
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        String s = in.nextLine().replace(":", "");
        backtracking(s);
        Collections.sort(list, (o1, o2) -> (o1 + o2).compareTo(o2 + o1));
        int index = list.indexOf(s);
        if (index != list.size() - 1){
            String s1 = list.get(index + 1);
            System.out.println(s1.substring(0, 2) + ":" + s1.substring(2, 4));
            return;
        }
        String s1 = list.get(0);
        System.out.println(s1.substring(0, 2) + ":" + s1.substring(2, 4));
    }
    public static void backtracking(String s){
        if (sb.length() == 4 && (sb.charAt(0) >= '3' || sb.charAt(2) >= '6')){
            return;
        }

        if (sb.length() == 4 && sb.charAt(0) < '3' && sb.charAt(2) < '6'){
            if (sb.charAt(0) == '2'){
                if (sb.charAt(1) <= '3'){
                    if (!list.contains(sb.toString())){
                        list.add(sb.toString());
                    }
                }
            }else {
                if (!list.contains(sb.toString())){
                    list.add(sb.toString());
                }
            }
            return;
        }
        for (int i = 0; i < s.length(); i++) {
            sb.append(s.charAt(i));
            backtracking(s);
            sb.delete(sb.length() - 1, sb.length());
        }
    }
}