9. Palindrome Number

Problem

Tags: Math

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 a palindrome while 123 is not.

Example 1:

Input: x = 121
Output: true
Explanation: 121 reads as 121 from left to right and from right to left.

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.

Constraints:

• -2^31 <= x <= 2^31 - 1

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

Code

C

// 9. Palindrome Number (11/15/53956)
// Runtime: 8 ms (84.94%) Memory: 6.07 MB (18.87%) 

bool isPalindrome(int x){
    // x 是負數，直接 false
    if(x < 0) return false;
    
    // y 用來存 x 的反轉數
    int64_t reversed = 0;
    
    // 從最後一位開始，每次將 reversed 左推後加上該位之值
    for(int64_t num = x; num > 0; num /= 10) {
        reversed = reversed * 10 + num % 10;
    }
    
    // 比較兩者是否相等
    return (int64_t)x == reversed;
}