leetcode 7. Reverse Integer

https://leetcode.com/problems/reverse-integer/

溢出处理，最简洁形式

class Solution {
public:
    int reverse(int x) {
        int res=0;
        while(x!=0){
            if(res>INT_MAX/10 || res<INT_MIN/10) return 0;
            res = res*10 + x%10;
            x/=10;
        }
        return res;
    }
};

利用unsigned，稍微不美观方式

class Solution {
public:
    int reverse(int x) {
        unsigned int val = abs(x);
        unsigned int result = 0;
    
        while (val) {
            if (result > ((unsigned int)(1 << 31) - 1) / 10)
                return 0;
    
            result = result * 10 + val % 10;
            val /= 10;
        }
    
        return (x > 0 ? result : (-1 * result));
    }
};

关于 INT_MAX 计算：

#include <iostream>

using namespace std;

int main()
{
    cout << "INT_MAX " << ((unsigned int)(1 << 31) - 1) << endl;
    cout << (1 << 31) << endl;
    cout << (unsigned int)(1 << 31) << endl;
    cout << "INT_MAX " << INT_MAX << endl;;
    cout << "INT_MIN " << INT_MIN << endl;;
    cout << "unsigned INT_MAX " << (unsigned int)INT_MAX << endl;;
    cout << "unsigned INT_MIN " << (unsigned int)INT_MIN << endl;;
    return 0;
}

输出结果

2^31 = 2 147 483 648

INT_MAX 2147483647
-2147483648
2147483648
INT_MAX 2147483647
INT_MIN -2147483648
unsigned INT_MAX 2147483647
unsigned INT_MIN 2147483648

linux宏定义

#define INT_MAX ((int)(~0U>>1))
#define INT_MIN (-INT_MAX - 1)
#define UINT_MAX (~0U)
#define LONG_MAX ((long)(~0UL>>1))
#define LONG_MIN (-LONG_MAX - 1)
#define ULONG_MAX (~0UL)
#define LLONG_MAX ((long long)(~0ULL>>1))
#define LLONG_MIN (-LLONG_MAX - 1)
#define ULLONG_MAX (~0ULL)

以INT_MAX为例说明：符号整型数，0取反得到全F，右移1位空出符号位，得到最大的符号整型数，无论它是16位平台还是32位平台，都是正确的，其他各个功能都类似。