錯題集。。

目錄

1、選擇題

2、 程式設計題

組隊競賽

排序子序列

倒置字元串

1、選擇題

2、 程式設計題

組隊競賽

【推導公式！！！】

import java.util.*;

public class Main {
    public static void main(String[] args) {
        //公式：arr.length - 2 *(i +1)
        //i屬于[0,n)
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] arr = new int[3 * n];
        for (int i = 0; i < arr.length; i++) {
            arr[i] = sc.nextInt();
        }
        Arrays.sort(arr);
        long sum = 0;
        for (int i = 0; i < n; i++) {
            sum += arr[arr.length - (2 * (i + 1))];
        }
        System.out.println(sum);
    }
}
           

排序子序列

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 + 1];
        for (int i = 0; i < n; i++) {
            arr[i] = sc.nextInt();
        }
        int i = 0;
        int count = 0;
        while (i < n) {
            if (arr[i] < arr[i + 1]) {
                while (i < n && arr[i] < arr[i + 1]) {
                    i++;
                }
                i++;
                count++;
            } else if (arr[i] == arr[i + 1]) {
                i++;
            } else {
                while (i < n && arr[i] > arr[i + 1]) {
                    i++;
                }
                i++;
                count++;
            }
        }
        System.out.println(count);
    }
}
           

倒置字元串​​​​​​​

import java.util.*;

public class Main {
    //1、整體資料逆置
    //2、每個單詞逆置
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String str = sc.nextLine();
        char[] arr = str.toCharArray();
        reverse(arr, 0, arr.length - 1);
        int len = arr.length;
        for (int i = 0; i < arr.length;) {
            int j = i;
            while (j < len && arr[j] != ' ') {
                j++;
            }
            if (j < len) {
                reverse(arr, i, j -1);//j走到空格
                i = j + 1;
            }else {
                reverse(arr, i, j -1);
                i = j;
            }
        }
        System.out.println(String.valueOf(arr));
    }

    private static void reverse(char[] arr, int start, int end) {
        while (start < end) {
            char c = arr[start];
            arr[start] = arr[end];
            arr[end] = c;
            start++;
            end--;
        }
    }
}