算法：回文数字9. Palindrome Number

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题目

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 

全部求余数解法

用上回得到的结果乘以10，加上x除以10余数，得到的最终结果，和x比较即可。

class Solution { public boolean isPalindrome(int x) { // check edge if (x < 0) { return false; } int reverse = 0; int copy = x; while (copy > 0) { int last = copy % 10; reverse = reverse * 10 + last; copy = copy / 10; } return reverse == x; } } 

折半求余数解法

实际上只要折半后就能得到结果，也就是x > reverse。
这里需要注意，除以10整除的数，都不行。

class Solution { public boolean isPalindrome(int x) { // check edge if (x < 0 || (x > 0 && x % 10 == 0)) { return false; } int reverse = 0; while (x > reverse) { int last = x % 10; reverse = reverse * 10 + last; x = x / 10; } return x == reverse || x == reverse / 10; } }