LeetCode - Easy - 9. Palindrome Number

小咪咪 2023-01-01 15:50 82阅读 0赞

## Topic ##

*  Math

## Description ##

[https://leetcode.com/problems/palindrome-number/][https_leetcode.com_problems_palindrome-number]

Determine whether an integer is a palindrome. An integer is a palindrome when it reads the same backward as forward.

**Follow up**: Could you solve it without converting the integer to a string?

**Example 1**:

Input: x = 121
    Output: true

**Example 2**:

Input: x = -121
    Output: false
    Explanation: From left to right, it reads -121. From right to left, it becomes 121-. Therefore it is not a palindrome.

**Example 3**:

Input: x = 10
    Output: false
    Explanation: Reads 01 from right to left. Therefore it is not a palindrome.

**Example 4**:

Input: x = -101
    Output: false

**Constraints**:

*  \-2³¹ <= x <= 2³¹ - 1

## Analysis ##

方法一：我写的，用一个10位整型数组存数位，然后首尾指针向中夹逼迭代数组元素过程中，判断所指元素的相等性，从而判断实参是否是回文数。

方法二：利用回文的轴对称性，也就是轴右半部分反转的数是否等于前半部分的数。

方法三：更方法二类似，一个数的如果是回文数，那么这数反转后，与原数相等。该方法并没有明显处理负数和溢出的情况的代码，是一个似非而是的方法。

## Submission ##

public class PalindromeNumber { 
    
    	// 方法一：我写的
    	public boolean isPalindrome1(int x) { 
    		if (x < 0)
    			return false;
    		int[] digits = new int[10];
    		int leftIndex = 0, rightIndex = -1;
    
    		while (x > 0) { 
    			digits[++rightIndex] = x % 10;
    			x /= 10;
    		}
    
    		while (leftIndex < rightIndex) { 
    			if (digits[leftIndex++] != digits[rightIndex--])
    				return false;
    		}
    
    		return true;//
    	}
    
    	// 方法二：
    	public boolean isPalindrome2(int x) { 
    		if (x < 0 || (x != 0 && x % 10 == 0))
    			return false;
    		int rev = 0;
    		while (x > rev) { 
    			rev = rev * 10 + x % 10;
    			x = x / 10;
    		}
    		return (x == rev || x == rev / 10);
    	}
    
    	// 方法三：
    	public boolean isPalindrome3(int x) { 
    		int palindromeX = 0;
    		int inputX = x;
    		while (x > 0) { 
    			palindromeX = palindromeX * 10 + (x % 10);
    			x = x / 10;
    		}
    		return palindromeX == inputX;
    	}
    
    }

## Test ##

import static org.junit.Assert.*;
    import org.junit.Test;
    
    public class PalindromeNumberTest { 
    
    	@Test
    	public void test() { 
    		PalindromeNumber obj = new PalindromeNumber();
    		assertTrue(obj.isPalindrome1(121));
    		assertTrue(obj.isPalindrome1(2147447412));
    		assertFalse(obj.isPalindrome1(-121));
    		assertFalse(obj.isPalindrome1(10));
    		assertFalse(obj.isPalindrome1(-101));
    		assertFalse(obj.isPalindrome1(Integer.MAX_VALUE));
    		
    		assertTrue(obj.isPalindrome2(121));
    		assertTrue(obj.isPalindrome2(2147447412));
    		assertFalse(obj.isPalindrome2(-121));
    		assertFalse(obj.isPalindrome2(10));
    		assertFalse(obj.isPalindrome2(-101));
    		assertFalse(obj.isPalindrome2(Integer.MAX_VALUE));
    		
    		assertTrue(obj.isPalindrome3(121));
    		assertTrue(obj.isPalindrome3(2147447412));
    		assertFalse(obj.isPalindrome3(-121));
    		assertFalse(obj.isPalindrome3(10));
    		assertFalse(obj.isPalindrome3(-101));
    		assertFalse(obj.isPalindrome3(Integer.MAX_VALUE));
    	}
    }

[https_leetcode.com_problems_palindrome-number]: https://leetcode.com/problems/palindrome-number/