【LeetCode】7. Reverse Integer

Reverse Integer

Reverse digits of an integer.

Example1: x = 123, return 321
Example2: x = -123, return -321

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Have you thought about this?

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).

 

（1）首先将x提升为64位num，若不然-INT_MIN将会溢出.

（2）将num取模10得到的每一位数字加入到返回值中，然后返回值乘10移位，得到数值上的结果y。

（3）判断y是否超过int的范围，若超过，则返回0

时间复杂度为：O(k), k为x的位数

class Solution {
public:
    int reverse(int x) {
        long long y = 0;
        long long num = x;  //-INT_MIN will out of range
        int sign = 1;
        if(num < 0)
        {
            sign = -1;
            num = -num;
        }

        //note that the range of integer is [-2147483648, 2147483647]
        //check invalid first
        while(num)
        {
            y = y*10 + num%10;
            num /= 10;
        }
        if(y > INT_MAX) 
        {
            if(y == INT_MAX+1 && sign == -1)
                return INT_MIN;
            else    //invalid set 0
                return 0;
        }
        return y*sign;  //valid
    }
};