选几种典型的加以说明

import java.io.BufferedInputStream;
import java.util.Scanner;

public class test {

    // 不返回串
    public static void reversePrint(String s, int i) {
        if (i != s.length()) {
            reversePrint(s, i + 1);
            System.out.print(s.charAt(i));
        }
    }
     // 返回串
    public static String reverseString(String s, int i) {
        if (i == s.length()) {
            return "";
        } else {
            String s1 = reverseString(s, i + 1);
            return s1 + s.charAt(i);
        }
    }
     // 返回串
    public static String reverseString1(String s) {
        int N = s.length();
        if (N <= 1) {
            return s;
        }
        String s1 = s.substring(0, N / 2);
        String s2 = s.substring(N / 2, N);
        return reverseString1(s2) + reverseString1(s1);
    }
     // 返回串,系统自带方法就是这样实现的，测试结果也是这种方法最快
    public static String reverseString2(String s) {
        int N = s.length() - 1;
        char[] c = s.toCharArray();
        for (int j = (N - 1) >> 1; j >= 0; --j) {
            int k = N - j;
            char ck = c[j];
            c[j] = c[k];
            c[k] = ck;
        }
        return new String(c);
    }
    public static void main(String[] args) {
        Scanner cin = new Scanner(new BufferedInputStream(System.in));
        String s = cin.next();
        cin.close();
        long l1 = System.nanoTime();
        reversePrint(s, 0); // 直接打印
        long e1 = System.nanoTime();
        System.out.println();
        System.out.println((e1-l1)+"ns");
        System.out.println();   
        
        long l2 = System.nanoTime();
        System.out.println(reverseString(s, 0)); // 直接递归到底层返回逆序回串
        long e2 = System.nanoTime();
        System.out.println((e2-l2)+"ns");
        System.out.println();
        
        long l3 = System.nanoTime();
        System.out.println(reverseString1(s)); // 二分递归到底层返回逆序串
        long e3 = System.nanoTime();
        System.out.println((e3-l3)+"ns");
        System.out.println();
        
        long l4 = System.nanoTime();
        System.out.println(reverseString2(s)); // 和java的StringBuilder一样的实现
        long e4 = System.nanoTime();
        System.out.println((e4-l4)+"ns");
        System.out.println();
    }
}

       



综上所述，最后一种方法最好，StringBuilder自带的reverse也是这样实现的


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