LeetCode - Easy - 7. Reverse Integer

今天药忘吃喽~ 2023-01-01 03:58 113阅读 0赞

## Topic ##

*  Math

## Description ##

[https://leetcode.com/problems/reverse-integer/][https_leetcode.com_problems_reverse-integer]

Given a 32-bit signed integer, reverse digits of an integer.

**Note**:

Assume we are dealing with an environment that could only store integers within the 32-bit signed integer range: \[−2³¹, 2³¹ − 1\]. For the purpose of this problem, assume that your function returns 0 when the reversed integer overflows.

**Example 1**:

Input: x = 123
    Output: 321

**Example 2**:

Input: x = -123
    Output: -321

**Example 3**:

Input: x = 120
    Output: 21

**Example 4**:

Input: x = 0
    Output: 0

**Constraints**:

*  \-2³¹ <= x <= 2³¹ - 1

## Analysis ##

经验教训：

判断溢出时的老路：

boolean sign = x < 0;
    ...
    if(sign && result > 0 || !sign && result < 0) {
    	return 0;
    }

但是用在本题就不好用了。

System.out.println(Integer.MAX_VALUE + 1);// -2147483648
    System.out.println(964632435 * 10);// 1056389758

`964632435 * 10`明明溢出了，却还是正数。猜测原因是`964632435 * 10`底层算法为10个964632435进行累加，验证代码如下：

int result = 0;
    for (int i = 0; i < 10; i++)
    	result += 964632435;
    System.out.println(result);// 1056389758

猜测正确。

## Submission ##

public class ReverseInteger { 
    	public int reverse(int x) { 
    		int result = 0, last = 0;
    		while (x != 0) { 
    			last = result;
    			result = result * 10 + x % 10;
    			if (result / 10 != last) { 
    				return 0;
    			}
    			x /= 10;
    		}
    		return result;
    	}
    
    	public static void main(String[] args) { 
    		System.out.println(Integer.MAX_VALUE + 1);// -2147483648
    		System.out.println(964632435 * 10);// 1056389758
    		int result = 0;
    		for (int i = 0; i < 10; i++)
    			result += 964632435;
    		System.out.println(result);// 1056389758
    	}
    }

## Test ##

import static org.junit.Assert.*;
    import org.junit.Test;
    
    public class ReverseIntegerTest { 
    
    	@Test
    	public void test() { 
    		ReverseInteger obj = new ReverseInteger();
    
    		assertEquals(321, obj.reverse(123));
    		assertEquals(-321, obj.reverse(-123));
    		assertEquals(21, obj.reverse(120));
    		assertEquals(0, obj.reverse(0));
    		assertEquals(0, obj.reverse(1534236469));
    	}
    }

[https_leetcode.com_problems_reverse-integer]: https://leetcode.com/problems/reverse-integer/