[leetcode]9. Palindrome Number 回文数

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

Example 1:

Input: 121
Output: true

Example 2:

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

Follow up:

Coud you solve it without converting the integer to a string?

 

题意：

判断是否为回文数

 

Solution1:  reverse the original number as reversedInt,  if reversedInt == original number,  it is a Palindrome 

      original number     reversedInt      isPalindrome()

        123                       321                flase

        121                        121                true

 

code

 /*
Time Complexity:  O(n), loop each digit of input number 
Space Complexity: O(1), allocate a variable temp
*/
class Solution {
    public boolean isPalindrome(int x) {
        // handle input is negative  like -121 or end with 0 
        if (x < 0 || (x != 0 && x % 10 == 0)) return false;

        int temp = x;
        long sum = 0;
        while (x != 0) {
            sum = sum * 10 + x % 10;
            x = x / 10;
        }
        return (int)sum == temp;
    }
}