面试中的算法（持续更新中）

• 给定一个数组a，长度为n，求数组中所有两个元素相加之和为N的小标i和j

• import java.util.*;
  
  public class Main {
  
      public static void main(String[] args) {
          Scanner sc = new Scanner(System.in);
          int n = sc.nextInt();
          int N = sc.nextInt();
          int[] arr = new int[n];
          Map map = new HashMap<>();
          for (int i=0;i

• 三个线程的名字分别为0,1,2，每个线程输出十次，按照012012012. . .的方式输出

• public class Main {
      //用来记录前一个数的值
      int x=-1;
  
      public static void main(String[] args){
          Main m = new Main();
  
          new Thread(m.new RunnableA("0")).start();
          new Thread(m.new RunnableB("1")).start();
          new Thread(m.new RunnableC("2")).start();
      }
  
      class RunnableA implements Runnable{
          private String name;
  
          public RunnableA(String name) {
              this.name = name;
          }
  
          @Override
          public void run(){
              for(int i=0;i<10;i++){
                  synchronized (Main.class){
                      if(x == 1 || x == 0){
                          try {
                              Main.class.wait();
                          } catch (InterruptedException e) {
                              e.printStackTrace();
                          }
                          i--;
                          continue;
                      } else {
                          System.out.print(name);
                          x = 0;
                          Main.class.notifyAll();
                      }
                  }
              }
          }
      }
  
      class RunnableB implements Runnable{
          private String name;
  
          public RunnableB(String name){
              this.name = name;
          }
  
          @Override
          public void run(){
              for(int i=0;i<10;i++){
                  synchronized (Main.class){
                      if(x == 1 || x == 2){
                          try {
                              Main.class.wait();
                          } catch (InterruptedException e) {
                              e.printStackTrace();
                          }
                          i--;
                          continue;
                      } else {
                          System.out.print(name);
                          x = 1;
                          Main.class.notifyAll();
                      }
                  }
              }
          }
      }
  
      class RunnableC implements Runnable{
          private String name;
  
          public RunnableC(String name){
              this.name = name;
          }
  
          @Override
          public void run(){
              for(int i=0;i<10;i++){
                  synchronized (Main.class){
                      if(x == 0 || x == 2){
                          try {
                              Main.class.wait();
                          } catch (InterruptedException e) {
                              e.printStackTrace();
                          }
                          i--;
                          continue;
                      } else {
                          System.out.print(name);
                          x = 2;
                          Main.class.notifyAll();
                      }
                  }
              }
          }
  
      }
  }

• 给定一个字符串，得到这个数组按顺序最小的字符串输出，最小的输出个数为s

• 例如：str=1584632   s=3   输出：132

• public class Main {
      public static void main(String[] args) {
          Scanner sc1 = new Scanner(System.in);
          Scanner sc2 = new Scanner(System.in);
  
          //输入一个字符串
          String str = sc1.nextLine();
          //得到剩余的最小的个数
          int s = sc2.nextInt();
  
          //将字符串变成 int[]
          int[] arr = new int[str.length()];
          for (int i=0;i map = new HashMap<>();
          List list = new ArrayList<>();
          for (int i=0;i map1 = new HashMap<>();
          for (int i=0;i> list1 = new ArrayList<>();
          Set> entries = map1.entrySet();
          for (Map.Entry e : entries){
              list1.add(e);
          }
          Collections.sort(list1, new Comparator>() {
              @Override
              public int compare(Map.Entry o1, Map.Entry o2) {
                  return o1.getValue() - o2.getValue();
              }
          });
  
          //排序完成后进行输出，如果首位为 0 那么跳过
          for (Map.Entry m : list1){
              if (m.getKey() != 0){
                  System.out.print(m.getKey());
              }
          }
      }
  }

• 比较版本号：第二个版本号大于第一个那么就返回true，否则返回false

• import java.util.*;
  
  public class Main {
      public static void main(String[] args) {
          Scanner sc = new Scanner(System.in);
          int n = sc.nextInt();
  
          for (int q = 0;qarr2[i]){
                          System.out.println("false");
                          break;
                      }
                  }
              }
          }
      }
  }

• 一个数的各个位的平方和得到一个数，如果得到的结果为1，则返回true，否则继续这行这个操作

• 例如：19
• 1^2+9^2=82
• 8^2+2^2=68
• 6^2+8^2=100
• 1^2+0^2+0^2=1
• 则返回true

• import java.util.*;
  
  public class Main {
  
      public static void main(String[] args) {
          Scanner sc = new Scanner(System.in);
          int n = sc.nextInt();
          for (int i = 0; i < n; i++) {
              int t = sc.nextInt();
              slove(t);
          }
          sc.close();
      }
  
      public static void slove(int t){
  
          Map map = new HashMap<>();
          map.put(t," ");
          int num = 0;
          int k = 0;
          while (t!=1){
              char[] arr = String.valueOf(t).toCharArray();
              num = 0;
              for (int j = 0; j < arr.length; j++) {
                  k = arr[j] - '0';
                  num+=k*k;
              }
              t = num;
              //如果存在当前key那么就不会得到1,输出false
              if(map.get(t)!=null){
                  System.out.println("false");
                  return;
              }else {
                  map.put(t," ");
              }
          }
          System.out.println("true");
      }
  }
  

• 数字和字符串进行归并，每四个数字后边跟一个字母，如果字母多余那么跟在后边

• import java.util.*;
  
  public class text {
  
      public static void main(String[] args) {
          Scanner scanner = new Scanner(System.in);
          String s1 = scanner.nextLine();
          String s2 = scanner.nextLine();
          String[] split1 = s1.split(" ");
          String[] split2 = s2.split(" ");
          if (split1.length == 0 && split2.length == 0) {
              System.out.println();
              return;
          }
          if (split1.length==0){
              System.out.println(s2);
              return;
          }
          if (split2.length==0){
              System.out.println(s1);
              return;
          }
  
          StringBuffer sb = new StringBuffer();
          int index = 0;
          for (int i=0;i

• 
• 、

• import java.util.*;
  
  public class Main {
  
      public static void main(String[] args) {
          Scanner sc = new Scanner(System.in);
          String s = sc.nextLine();
          //\\s表示   空格,回车,换行等空白符,
          //+号表示一个或多个的意思
          String[] str = s.split(",\\s+");
          Map map = new HashMap<>();
          int sum = 0;
          for (int i = 0; i < str.length; i++) {
              int value = Integer.valueOf(str[i]);
              if (map.containsKey(-value) && map.get(-value) > 0) {
                  int count = map.get(-value);
                  map.put(-value, count - 1);
                  sum++;
              } else {
                  int count = map.getOrDefault(value, 0);
                  map.put(value, count + 1);
              }
          }
          System.out.println(sum);
       }
  }
  

• 实现一个数组的最长递增序列

• import java.util.*;
  
  public class text5 {
      public static void main(String[] args) {
          Scanner sc = new Scanner(System.in);
          String[] str= sc.nextLine().split(" ");
          int[] arr = new int[str.length];
          for (int i = 0; i < str.length; i++){
              arr[i] = Integer.parseInt(str[i]);
          }
          int max = 0, count = 0, ss = 0, x = 0, y = 0;
          for (int i = 0; i < str.length - 1; i++) {
              y = i + 1;//判断是否递增，是的话count++;
              if (arr[i + 1] > arr[i]) {
                  count++;
                  if (count > max) {
                      max = count;
                      ss = x;
                  }
              } else {
                  count = 0;
                  x = y;//不连续递增，则索引改变为下一个目标
              }
          }
          for (int i = ss; i <= ss + max; i++) {
              if (i == (ss + max)) {
                  System.out.print(arr[i]);
              } else {
                  System.out.print(arr[i] + " ");
              }
          }
      }
  }
  

• 归并排序

• 
  import java.util.*;
  
  public class Main {
  
      public static void main(String[] args) {
          Scanner sc = new Scanner(System.in);
          int n = sc.nextInt();
          int[] a = new int[n];
          for (int i=0;i

• 分饼干问题：没个人要达到满意，每个人至少分得一个饼干

• import java.util.*;
  
  public class Main {
      public static void main(String [] args) {
          Scanner sc = new Scanner(System.in);
          int n = sc.nextInt();
          int[] arr = new int[n];
          for (int i=0;i arr[i-1]){
                  sum[i] = sum[i-1]+1;
              }
          }
          for(int i = sum.length-2;i>=0;i--){
              if(arr[i] > arr[i+1]){
                  sum[i] = Math.max(sum[i],sum[i+1]+1);
              }
          }
          int num = 0;
          for(int i = 0;i < arr.length;i++){
              num += sum[i];
          }
          return num;
      }
  }
  

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