LeetCode OJ:Palindrome Number

Palindrome Number

 

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

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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.

算法思想：

按照题意，一前一后进行比较

class Solution {
public:
    bool isPalindrome(int x) {
        if (x < 0) return false;
        int div = 1;
        while (x / div >= 10) {
            div *= 10;
        }        
        while (x != 0) {
            int l = x / div;
            int r = x % 10;
            if (l != r) return false;
            x = (x % div) / 10;
            div /= 100;
        }
        return true;
    }
};

还有一个递归算法，感觉很巧妙

class Solution {
public:
    bool isPalindrome(int x,int &y){
        if (x < 0) return false;
        if (x == 0) return true;
        if (isPalindrome(x/10, y) && (x%10 == y%10)) {
            y /= 10;
            return true;
        } else {
            return false;
        }
    }
    bool isPalindrome(int x) {
        return isPalindrome(x,x);
    }
};