1 #include<cctype> 2  3 class Solution { 4 public: 5     int atoi(const char *str) { 6         int signal = 0;          /*标记正负，还未出现判定正负的标志时，记为0*/ 7         long long tmp = 0; 8  9         if(*str == ‘\0‘)10             return 0;11         while(*str != ‘\0‘)12         {13             if(*str == ‘ ‘ && signal != 0)  /*表明空格前以出现过数字*/14             {15                 break;16             }17             if(*str == ‘+‘)18             {19                 if(signal == 0)     /*出现正号之前没有出现过数字*/20                     signal = 1;21                 else22                     break;23             }24             if(*str == ‘-‘)25             {26                 if(signal == 0)27                     signal = -1;28                 else29                     break;30             }31             if(*str >= ‘0‘ && *str <= ‘9‘)32             {33                 if(signal == 0)      /*出现数字之前没出现正负号*/34                     signal = 1;35                 tmp = tmp*10+((*str)-‘0‘);36             }37             if((tmp*signal)>2147483647)38                 return INT_MAX;39             if((tmp*signal)<-2147483648)40                 return INT_MIN;41             if((!isdigit(*str)) && (*str!=‘+‘) && (*str!=‘-‘) && (*str!=‘ ‘))42             {43                 break;44             }45             str++;46         }47         if(signal == 0)48             return 0;49         else 50             return tmp*signal;51     }52 };