面试题49：把字符串转换成整数（atoi）

需要考虑的问题都已在程序中注释

bool isValid;

int StrToInt(const char* str)
{
    isValid = false;
    //不合法情形1：空指针
    if (str == NULL)    
        return 0;
    
    //不合法情形2：内容为“”
    if (*str == '')
        return 0;

    const char *pData = str;
    bool isNegative = false;
    if (*pData == '+')
    {
        isNegative = false;    //是否是负数
        pData++;
        //不合法情形3：内容为+
        if (*pData == '')
            return 0;
    }    
    else if (*pData == '-')
    {
        isNegative = true;
        pData++;
        //不合法情形3：内容为-
        if (*pData == '')
            return 0;
    }
        
    long long num = 0;    //设为long long检查越界
    while (*pData != '')
    {
        //不合法情形5：存在非数字
        if (*pData < '0' || *pData > '9')
            return 0;
        else
        {
            num = num * 10 + (*pData - '0');
        }
        pData++;
    }

    //不合法情形6：越界
    num = isNegative ? 0 - num : num;
    //注意一定要加(signed int)
    if (isNegative && num < (signed int)0x80000000 || !isNegative && num > 0x7FFFFFFF)
        return 0;

    isValid = true;
    return num;
}

带测试的完整代码：

#include<iostream>
using namespace std;

bool isValid;

int StrToInt(const char* str)
{
    isValid = false;
    //不合法情形1：空指针
    if (str == NULL)    
        return 0;
    
    //不合法情形2：内容为“”
    if (*str == '')
        return 0;

    const char *pData = str;
    bool isNegative = false;
    if (*pData == '+')
    {
        isNegative = false;    //是否是负数
        pData++;
        //不合法情形3：内容为+
        if (*pData == '')
            return 0;
    }    
    else if (*pData == '-')
    {
        isNegative = true;
        pData++;
        //不合法情形3：内容为-
        if (*pData == '')
            return 0;
    }
        
    long long num = 0;    //设为long long检查越界
    while (*pData != '')
    {
        //不合法情形5：存在非数字
        if (*pData < '0' || *pData > '9')
            return 0;
        else
        {
            num = num * 10 + (*pData - '0');
        }
        pData++;
    }

    //不合法情形6：越界
    num = isNegative ? 0 - num : num;
    //注意一定要加(signed int)
    if (isNegative && num < (signed int)0x80000000 || !isNegative && num > 0x7FFFFFFF)
        return 0;

    isValid = true;
    return num;
}

void Test(const char *testNum)
{
    int result = StrToInt(testNum);
    if (testNum == NULL)
    {
        cout << "char * is Null" << endl;
        cout << "input is not vaild" << endl;
        cout << endl;
        return;
    }    
    cout << "Test string is: " << testNum << endl;
    if (isValid)
        cout << result << endl;
    else
        cout << "input is not vaild" << endl;
    cout << endl;
    
}

int main()
{
    Test(NULL);

    Test("");

    Test("123");

    Test("+123");

    Test("-123");

    Test("1a33");

    Test("+0");

    Test("-0");

    Test("+");

    Test("-");
    
    //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");

    system("pause");

    return 0;
}

itoa实现，最后别忘加=''

void itoa(int num, char *str)
{
    int power = 1;
    int j = num;

    while (j / 10 != 0)
    {
        power *= 10;
        j /= 10;
    }
    
    while (num != 0)
    {
        *str++ = '0' + (num / power);
        num %= power;
        power /= 10;
    }
    *str = '';
}