7. Reverse Integer

题目大意： 将Integer类型的数字 反转输出

Reverse digits of an integer.

Example1: x = 123, return 321
Example2: x = -123, return -321

• 第一种方法：将数字转为string类型，通过StringBuilder函数转换。

public class ReverseInteger {
    public static int reverse(int x) {
        String str = String.valueOf(x);
        StringBuilder reverse = new StringBuilder(str).reverse();
        return Integer.valueOf(reverse.toString());
    }
    public static void main(String args[] ){
        int reverse = ReverseInteger.reverse(123);
        System.out.println(reverse);
    }
}

• 第二种方法：通过原始的方法 。注意 int 类型 下标越界问题

    public static int reverse(int x){
        if (x >= 0 && x <= 2147483647) {
            int a = x;
            int b = x / 10;
            int c = x % 10;
            int result = c;
            while (b != 0) {
                if (result > 2147483647/10) {
                    return 0;
                }
                result *= 10;
                c = b % 10;
                b = b / 10;
                if(result > 2147483647 - c)
                    return 0;
                result += c;
            }
            return result;
        }else if (x < 0 && x >= -2147483648){
            x = Math.abs(x);
            int b = x / 10;
            int c = x % 10;
            int result = c;
            while (b != 0) {
                if (result > 2147483647/10) {
                    return 0;
                }
                result *= 10;
                c = b % 10;
                b = b / 10;
                if(result > 2147483647 - c)
                    return 0;
                result += c;
            }
            return result;
        }else {
            return 0;
        }

    }