Leetcode 0009: Palindrome Number

题目描述：

Given an integer x, return true if x is palindrome integer.
An integer is a palindrome when it reads the same backward as forward. For example, 121 is palindrome while 123 is not.

Note:

Input: x = 121
Output: true

Example 1:

Input: s = “42”
Output: 42
Explanation: The underlined characters are what is read in, the caret is the current reader position.
Step 1: “42” (no characters read because there is no leading whitespace)
Step 2: “42” (no characters read because there is neither a ‘-’ nor ‘+’)
Step 3: “42” (“42” is read in)
The parsed integer is 42.
Since 42 is in the range [-2³¹, 2³¹ - 1], the final result is 42.

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

Example 5:

Input: s = “-91283472332”
Output: -2147483648
Explanation:
Step 1: “-91283472332” (no characters read because there is no leading whitespace)
Step 2: “-91283472332” (’-’ is read, so the result should be negative)
Step 3: “-91283472332” (“91283472332” is read in)
The parsed integer is -91283472332.
Since -91283472332 is less than the lower bound of the range [-2³¹, 2³¹ - 1], the final result is clamped to -2³¹ = -2147483648.

Constraints:

-2³¹ <= x <= 2³¹ - 1

Time complexity: O(1)
只使用int 记录：

1. 若x < 0，一定不是回文数，返回false
2. 借鉴Leetcode 0007: Reverse Integer一个正整数的字符串表示是回文串，当且仅当它的逆序和它本身相等。

class Solution {
    
    
    public boolean isPalindrome(int x) {
    
    
        if(x < 0 ) return false;
        int res = 0,y = x;
        while(x!= 0){
    
    
            res = res*10 + x%10;
            x/=10;
        }
        return res == y;
    }
}