• String翻转

1. 要求将字符串指定的部分进行翻转

public class StringHomework {
    public static void main(String[] args) {
        // 要求将字符串指定的部分进行翻转
        // 例如:abcdef ---> a edcb f   1, 4
        System.out.print("转换前: ");
        String s = "abcdef";
        String s1 = null;
        System.out.println(s);
        try {
            System.out.println("=========");
            System.out.print("转换后: ");
            System.out.println(StringHomework.reverse(s, 1, 4));
        } catch (NullPointerException e) {
            System.out.println(e.getMessage() + "传入的字符串为null！");
        }

    }

    public static String reverse(String str, int start, int end) throws NullPointerException{
        // 先将字符串转换为字符数组
        char[] chars = str.toCharArray();

        // 判断 str 是否合法
        if (str.isEmpty()) {
            throw new RuntimeException("传入的字符串数据不合法!");
        }

        // 判断 start 和 end 是否合法
        if (start < 0 || end > chars.length || start > end) {
            throw new RuntimeException("传入的起始位置数据不合法!");
        }

        // 进行数组翻转, 提取出要反转的部分
        char[] chars1 = new char[end - start + 1];
        System.arraycopy(chars, start, chars1, 0, chars1.length);
        // return new String(chars1);
        // 翻转 chars1 字符数组
        for (int i = 0; i < chars1.length / 2; i++) {
            char temp = chars1[i];
            chars1[i] = chars1[chars1.length - i - 1];
            chars1[chars1.length - i - 1] = temp;
        }
        // return new String(chars1);
        // 将原数组数据替换
        System.arraycopy(chars1, 0, chars, start, chars1.length);
        return new String(chars);
    }
}

运行结果：