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 ;

     //不合法情形2：内容为“”
     if (*str == '\0')
         return ;

     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 == '\0')
             return ;
     }
     else if (*pData == '-')
     {
         isNegative = true;
         pData++;
         //不合法情形3：内容为-
         if (*pData == '\0')
             return ;
     }

     long long num = ;    //设为long long检查越界
     while (*pData != '\0')
     {
         //不合法情形5：存在非数字
         if (*pData < '' || *pData > '')
             return ;
         else
         {
             num = num *  + (*pData - '');
         }
         pData++;
     }

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

     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 ;

     //不合法情形2：内容为“”
     if (*str == '\0')
         return ;

     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 == '\0')
             return ;
     }
     else if (*pData == '-')
     {
         isNegative = true;
         pData++;
         //不合法情形3：内容为-
         if (*pData == '\0')
             return ;
     }

     long long num = ;    //设为long long检查越界
     while (*pData != '\0')
     {
         //不合法情形5：存在非数字
         if (*pData < '' || *pData > '')
             return ;
         else
         {
             num = num *  + (*pData - '');
         }
         pData++;
     }

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

     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("");

     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 ;
 }

itoa实现，最后别忘加='\0'

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

     while (j /  != )
     {
         power *= ;
         j /= ;
     }

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