LeetCode_String to Integer (atoi)

Implement atoi to convert a string to an integer.

int atoi (const char * str);
Convert string to integer
Parses the C-string str interpreting its content as an integral number, which is returned as a value of type int.

The function first discards as many whitespace characters (as in isspace) as necessary until the first non-whitespace character is found. Then, starting from this character, takes an optional initial plus or minus sign followed by as many base-10 digits as possible, and interprets them as a numerical value.

The string can contain additional characters after those that form the integral number, which are ignored and have no effect on the behavior of this function.

If the first sequence of non-whitespace characters in str is not a valid integral number, or if no such sequence exists because either str is empty or it contains only whitespace characters, no conversion is performed and zero is returned.

class Solution {
public:
    int atoi(const char *str) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
         int len = strlen(str) ;
         int i =0;
         long long result = 0;
         int flag = 1, b= 0;
         int numjia = 0;
         int numjian = 0;
     
         while(i<len && str[i] == ' ') i++;
        
         
         for(; i< len ; i++)
         {
            switch(str[i])
            {
                case '+': numjia++;break;
                case '-': numjian++; flag =-1;break;
                case '0':
                case '1':
                case '2':
                case '3':
                case '4':
                case '5':
                case '6':
                case '7':
                case '8':
                case '9': result+=str[i] - '0';result*=10;break;
                default: b = 1;break;
            }
            if(b==1) break;
            if(numjia>1 ||numjian>1)
                return 0;
         }
         
    
          result = result/10;
          result *= flag;
          
          if(result > INT_MAX) return INT_MAX;
          if(result < INT_MIN) return INT_MIN;
          return result;
    }
};

 重写后：

class Solution {
public:
    int atoi(const char *str) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
   
        if(str == NULL ) return 0;
        
        int len = strlen(str);
        long long res = 0;
        int cur = 0, flag = 1;
        
        while(cur<len && str[cur] == ' ') ++cur;
        
        if( str[cur] == '-' || str[cur] == '+'){
            if(str[cur] == '-')
              flag = -1;
            cur++;
        }
        
        while(cur < len){
            int c = str[cur] - '0';
            if(c<0 || c > 9)
                break;
            res = c + res*10; 
            ++cur;
        }
        res *= flag ;
            
        if(res > INT_MAX)
            return INT_MAX;
        if(res < INT_MIN)
          return INT_MIN;
          
        return res;
    }
};