9. Palindrome Number

https://leetcode.com/problems/palindrome-number/

Problem

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?

Solution

1. Using to_string

class Solution {
public:
    bool isPalindrome(int x) {
        const string str = to_string(x);
        for (int i = 0; i < str.size() / 2; ++i) {
            if (str[i] != str[str.size() - 1 - i]) return false;
        }
        return true;
    }
};

2. $O(1)$ Space without converting to string

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

• #math

• #string

• #important