题解 | 字符串操作

import java.util.Scanner;

// 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 sc = new Scanner(System.in);
        int n = sc.nextInt();
        int m = sc.nextInt();
        String s = sc.next();
        StringBuilder sb = new StringBuilder(s);
        for (int i = 0; i < m; i++) {
            int l = sc.nextInt() - 1; // 转换为0基索引
            int r = sc.nextInt() - 1; // 转换为0基索引
            char c1 = sc.next().charAt(0);
            char c2 = sc.next().charAt(0);
            for (int p = l; p <= r; p++) {
                if (sb.charAt(p) == c1) {
                    sb.setCharAt(p, c2); // 直接用StringBuilder的setCharAt方法更清晰
                }
            }
        }
        System.out.println(sb.toString());
    }
}