65. Reverse Integer &&　Palindrome Number

Here are some good questions to ask before coding. Bonus points for you if you have already thought through this!

If the integer‘s last digit is 0, what should the output be? ie, cases such as 10, 100.

Did you notice that the reversed integer might overflow? Assume the input is a 32-bit integer, then the reverse of 1000000003 overflows. How should you handle such cases?

Throw an exception? Good, but what if throwing an exception is not an option? You would then have to re-design the function (ie, add an extra parameter).

思路： 注意符号，溢出。

class Solution {
public:
    int reverse(int x) {
        int tag = 1;
        if(x < 0){
            tag = -1;
            x *= -1;
        }
        int k = 0, y = 0;
        while(x > 0){
            y = y * 10 + (x % 10);
            x /= 10;
        }
        if(y < 0) printf("Overflow!\n");
        return (y * tag);
    }
};

Palindrome Number

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

click to show spoilers.

class Solution {
public:
    bool isPalindrome(int x) {
        if(x < 0) return false;
        int v = x, y = 0;
        while(v > 0){
            y = y * 10 + (v % 10);
            v /= 10;
        }
        if(y == x) return true;
        return false; // 注意：溢出时，也肯定是 false！
    }
};