LeetCode::Reverse Integer C語言

題目

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?

For the purpose of this problem, assume that your function returns 0 when the reversed integer overflows.

Update (2014-11-10):
Test cases had been added to test the overflow behavior.

分析

基本的解法很直觀，就是不斷的模10，一個一個數字的翻轉。

我們要小心overflow，判斷的方法是和climits頭文件裏定義的INT_MIN或者INT_MAX比較，而且要在output增長前比較，不然output增長後就直接溢出了，數字無意義了。

結果顯示，效率也算是主流了，8ms

C語言代碼

#include <stdio.h>

#include <stdlib.h>

#include <climits>

#define BASE 10

int reverse(int x)

{

    if ( 0==x )

    {

        return x;

    }

    const int MAX = INT_MIN;

    int sign; //-1 for negative or 1 for positive

    if (x>0)

    {

        sign = 1;

        x = -x;

    }else

    {

        sign = -1;

    }

    int output = 0;

    while (0 != x)

    {

        int tmp = x % BASE;

        if (output < (MAX-tmp)/BASE)

        {

            //overflows

            return 0;

        }else

        {

            output = output * BASE + tmp;

        }

        x = (x - tmp)/BASE;

    }

    return sign*abs(output);

}

int main()

{

    int input = 21464;

    printf("The input is %d, and its reverse is %d\n", input, reverse(input));

}