LeetCode: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.

##:关键在于如何取出数字的位数！

Solution:

public class Solution {
    public boolean 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;
    }
}

另外一种巧妙的递归方法：

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);
}