《剑指offer》第六十七题：把字符串转换成整数

// 面试题67：把字符串转换成整数
// 题目：请你写一个函数StrToInt，实现把字符串转换成整数这个功能。当然，不
// 能使用atoi或者其他类似的库函数。

#include <cstdio>

long long StrToIntCore(const char* str, bool minus);

enum Status { kValid = 0, kInvalid };
int g_nStatus = kValid;

int StrToInt(const char* str)
{
    long long num = 0;
    g_nStatus = kInvalid;

    if (str != nullptr && *str != '\0')  //处理字符串为空指针或""
    {
        bool minus = false; //负号标志
        if (*str == '+')
            ++str;
        else if (*str == '-')
        {
            ++str;
            minus = true;
        }

        if (*str != '\0')  //处理单符号问题
            num = StrToIntCore(str, minus);
    }
    return (int)num;
}

long long StrToIntCore(const char* digit, bool minus)
{
    long long num = 0;

    while (*digit != '\0')
    {
        if (*digit >= '0' && *digit <= '9')
        {
            int flag = minus ? -1 : 1;
            num = num * 10 + flag * (*digit - '0');

            if ((!minus && num > 0x7FFFFFFF)  //溢出
                || (minus && num < (signed int)0x80000000))
            {
                num = 0;
                break;
            }

            ++digit;
        }
        else  //无效字符串
        {
            num = 0;
            break;
        }
    }

    if (*digit == '\0')
        g_nStatus = kValid;

    return num;
}

// ====================测试代码====================
void Test(const char* string)
{
    int result = StrToInt(string);
    if (result == 0 && g_nStatus == kInvalid)
        printf("the input %s is invalid.\n", string);
    else
        printf("number for %s is: %d.\n", string, result);
}

int main(int argc, char* argv[])
{
    Test(nullptr);

    Test("");

    Test("123");

    Test("+123");

    Test("-123");

    Test("1a33");

    Test("+0");

    Test("-0");

    //有效的最大正整数, 0x7FFFFFFF
    Test("+2147483647");

    Test("-2147483647");

    Test("+2147483648");

    //有效的最小负整数, 0x80000000
    Test("-2147483648");

    Test("+2147483649");

    Test("-2147483649");

    Test("+");

    Test("-");

    return 0;
}

分析：考虑各种意外输入。

class Solution {
public:
    enum Status { kValid = 0, kInvalid };
    int g_nStatus = kValid;
    
    int StrToInt(string str) {
        
        long long num = 0;
        g_nStatus = kInvalid;
        int length = str.length();
        
        if (!str.empty())
        {
            int current = 0;
            bool minus = false;
            if (str[current] == '+')
                ++current;
            else if (str[current] == '-')
            {
                ++current;
                minus = true;
            }
            
            if (current != length)
                num = StrToInt(str, current, minus);
        }
        return (int)num;
    }
    
    long long StrToInt(string digit, int current, bool minus)
    {
        long long num = 0;
        int length = digit.length();
        
        int i = 0;
        for (i = current; i < length; ++i)
        {
            if (digit[i] <= '9' && digit[i] >= '0')
            {
                int flag = minus? -1 : 1;
                num = num * 10 + flag *(digit[i] - '0');
                
                if ((!minus && num > 0x7FFFFFFF)
                   || (minus && num < (signed int)0x80000000))
                {
                    num = 0;
                    break;
                }
            }
            else
            {
                num = 0;
                break;
            }
        }
        
        if (i == length)
            g_nStatus = kValid;
        
        return num;
    }
};