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?

如果可以用string，那么就可以

public boolean isPalindrome(int x) {
        String input  = String.valueOf(x);
        return Objects.equals(input, new StringBuilder(input).reverse().toString());
    }

　　

笨方法1：

Reverse integer 然后和原数字对比，如果对就是true。

class Solution {
    public boolean isPalindrome(int x) {
        boolean a = false;
        int res=0;
        int med = x;
        if(x<0||x%10==0){
            a =  false;
        }
        while(med>0){
            res = res*10+ med%10;
            med/=10;
        }
        if(x==res)
            a=true;
        return a;
      
    }
}

方法2：

只用取出后一半的数字，和前一半的数字进行对比，如果相等则返回true

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

　　这种方法比上面的理论上快一倍。