LeetCode Java First 400 题解-009

Palindrome Number    Easy

Determine whether an integer is a palindrome. Do this without extra space.

Some hints:

Could negative integers be palindromes? (ie, -1)

If you are thinking of converting the integer to string, note the restriction of using extra space.

You could also try reversing an integer. However, if you have solved the problem "Reverse Integer", you know that the reversed integer might overflow. How would you handle such case?

There is a more generic way of solving this problem.

public boolean isPalindrome(int x) {

    if (x < 0 || (x != 0 && x % 10 == 0)) return false;

    int rev = 0;

    while (x > rev) {

        rev = rev * 10 + x % 10;

        x = x / 10;

    }

    return (x == rev || x == rev / 10);

}

思路：先判定特殊情况：负和个位数。如果够长，则把后面一半逆位反过来（止于逆反数大于前面一半时）逆反数（或逆反数/10）和前面一半对比即可。