数学

判断素数

import java.util.Scanner;
public class LC {
 public static void main(String[] args) {
     Scanner sc = new Scanner(System.in);
     long num = sc.nextLong();
     if(num > 0) {
         //k为num的正平方根
         int k = (int)Math.sqrt(num);
         //从2开始，到k，如果num能够整除i，说明num能被除了1和它本身的数整除，不是素数
         for (int i = 2; i <= k; i++) {
             if (num % i == 0) {
                 System.out.println(false);
                 break;
             }
         }
     }
     System.out.println(true);
 }
}

素数的个数

import java.util.Scanner;
class Solution {
 public static void main(String[] args) {
     Scanner sc = new Scanner(System.in);
     int len = sc.nextInt();
     int[] nums = new int[len];

     long result = 0;
     //不断除以2就是素数的个数
     for (int i = 0; i < nums.length; i++) {
         nums[i] = sc.nextInt();
         result += nums[i] / 2;
     }
     System.out.println(result);
 }
}

字符串加法：https://leetcode-cn.com/problems/add-binary/

class Solution {
 public String addBinary(String a, String b) {
     //指向字符串尾部
     int len1 = a.length()-1;
     int len2 = b.length()-1;

     StringBuilder result = new StringBuilder();
     int carry = 0;

     //从尾部开始相加
     while(len1 >=0 || len2 >= 0 || carry != 0){
         if(len1 >= 0){
             carry += a.charAt(len1--) - '0';
         }
         if(len2 >= 0){
             carry += b.charAt(len2--) - '0';
         }
         //二进制相加
         result.append(carry%2);
         carry /= 2;
     }
     return result.reverse().toString();
 }
}

class Solution {
 public int majorityElement(int[] nums) {
     //摩尔投票法：相同就相消
     int count = 0;
     int result = 0;
     for(int i : nums){
         if(count == 0){
             result = i;
             count = 1;
         }else if(result == i){
             count++;
         }else{
             count--;
         }
     }
     return result;
 }
}

class Solution {
 public int trailingZeroes(int n) {
     //只有2和5相乘的结果才会有0，只需要考虑5即可
     int result = 0;
     //大于等于5时，不断除以5并叠加
     while(n >= 5){
         result += n/5;
         n /= 5;
     }
     return result;
 }
}