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.

判断一个数是不是回文。核心：回文意味着对称，所以只要能验证这个数是不是对称的即可。

方法一：

public static boolean isPalindrome(int x) {
    if(x < 0) {
        return false;
    }
    if(x >= 0 && x < 10) {
        return true;
    }
    String str = x + "";
    int preIndex = 0;
    int tailIndex = str.length() - 1;
    while(tailIndex >= preIndex) {
        //验证首尾指针指向的数字是否相等
        if(str.charAt(tailIndex) != str.charAt(preIndex)) {
            return false;
        }
        tailIndex--;
        preIndex++;
    }
    return true;
}

方法二：

public static boolean isPalindromeV2(int x){
    if(x < 0)
        return false;
    if(x == 0)
        return true;
    if(x % 10 == 0)
        return false;
    int ret = 0;
    while (x > ret) {
        //反序重建x
        ret = ret * 10 + x % 10;
        if(ret != x)
            x /= 10;
    }
    return x == ret;
}