9. Palindrome Number

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?

M1: 转换成string

time: O(n), space: O(n)

class Solution {
    public boolean isPalindrome(int x) {
        String s = Integer.toString(x);
        int i = 0, j = s.length() - 1;
        while(i <= j) {
            if(s.charAt(i) != s.charAt(j))
                return false;
            i++;
            j--;
        }
        return true;
    }
}

M2: 把x反转过来rev，最后如果x=rev，说明是palindrome

time: O(length of x), space: O(1)

class Solution {
    public boolean isPalindrome(int x) {
        if(x < 0) return false;
        int rev = 0;
        int xx = x;
        while(xx > 0) {
            rev = rev * 10 + xx % 10;
            xx /= 10;
        }
        return x == rev;
    }
}