代码模板

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner s = new Scanner(System.in);
        int n = s.nextInt();
        int k = s.nextInt();

        int[] a = new int[n];
        for (int i = 0; i < n; i++) {
            a[i] = s.nextInt();
        }
    }
}

快排模板

    private static void quickSort(int[] q, int l , int r) {
        // 递归终止条件
        if (l >= r) return;

        // 取分界点
        int x = q[l];
        int i = l - 1;
        int j = r + 1;
        while (i < j) {
            while (q[++i] < x);
            while (q[--j] > x);
            if (i < j) {
                int tmp = q[i];
                q[i] = q[j];
                q[j] = tmp;
            }
        }
        // 递归交换左右两个区间
        quickSort(q, l, j);
        quickSort(q, j + 1, r);
    }

归并排序

    public static void mergeSort(int[] q, int l, int r) {
        if (l >= r) return;

        int mid = l + r >> 1;
        // 递归对左右半边处理
        mergeSort(q, l, mid);
        mergeSort(q, mid + 1, r);

        // 排序，归并
        int i = l;
        int j = mid + 1;
        int k = 0;
        while (i <= mid && j <= r) 
            if (q[i] <= q[j]) tmp[k++] = q[i++];
            else tmp[k++] = q[j++];

        while (i <= mid) tmp[k++] = q[i++];
        while (j <= r) tmp[k++] = q[j++];

        // 存回原序列中
        for (i = l, j = 0; i <= r; i++, j++) q[i] = tmp[j]; 
    }

}

二分查找

索引二分查找

    public static void binarySearch(int[] q, int k) {
        // 查询k左边界
        int l = 0;
        int r = q.length - 1;
        while (l < r) {
            int mid = l + r >> 1;
            if (q[mid] >= k) r = mid;
            else l = mid + 1;
        }

        if (q[l] != k) { System.out.println("-1 -1"); return; }
        System.out.print("" + l + " ");

        // 查询k右边界
        l = 0; 
        r = q.length - 1;
        while (l < r) {
            int mid = l + r + 1 >> 1;
            if (q[mid] <= k) l = mid;
            else r = mid - 1;
        }

        System.out.println("" + l);
    }

值域二分查找

public class Main {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        double n = scan.nextDouble();

        double l = -10000;
        double r = 10000;
        // 满足6位小数精度
        while (r - l > 1e-8) {
            double mid = (l + r) / 2.0;
            if (mid * mid * mid >= n) r = mid;
            else l = mid;
        }

        System.out.println(String.format("%.6f", l));
    }
}

大精度相加

    public static String add(String a, String b) {
        int i = a.length() - 1;
        int j = b.length() - 1;
        int t = 0;
        StringBuilder sb = new StringBuilder();
        while (i >=0 || j >= 0 || t != 0) {
            if (i >= 0) t += a.charAt(i--) - '0';
            if (j >= 0) t += b.charAt(j--) - '0';
            sb.append(t % 10);
            t /= 10;
        }

        return sb.reverse().toString();
    }

前缀和

    public static void main(String[] args)  {
        Scanner s = new Scanner(System.in);
        int n = s.nextInt();
        int m = s.nextInt();

        int[] num = new int[n];
        for (int i = 0; i < n ; i++) {
            num[i] = s.nextInt();
        }

        // 求前缀和
        int[] sum = new int[n + 1];
        for (int i = 1; i <= n; i++) {
            sum[i] = sum[i - 1] + num[i - 1];
        }

        for (int i = 0; i < m; i++) {
            int l = s.nextInt();
            int r = s.nextInt();
            int res = sum[r] - sum[l - 1];
            System.out.println(res);
        }

    }