学步园

题目：Implement atoi to convert a string to an integer.

思路：需要注意一些特殊的输入

class Solution {
public:
    int atoi(const char *str) 
    {
       int len = strlen(str);
        int i,k;
        int64_t j = 1;
        int result = 0;
        bool positive = true;
        int max_or_min = 0;
        for(k=0; k<len; k++)
        {
          if (str[k] != ' ')
          {
             if(str[k] == '-')
             {
                positive = false;
                k++;
             } 
             else if (str[k] == '+')
                k++;
             break;
          }
        }
       
        for(i=len-1; i>=k; i--)
        {
           if ((str[i]-'0')>=0 && (str[i]-'0')<=9)
           {
              if (positive)
              {
                 if (uint64_t(result+(str[i]-'0')*j) >= INT_MAX)
                 {
                    max_or_min = INT_MAX;
                    j = 1;
                    continue;
                 }
                 result += (str[i]-'0')*j;
              }
              else
              {
                  if (result + (-1)*(str[i] - '0')*j <= INT_MIN)
                 {
                    max_or_min = INT_MIN;
                    j = 1;
                    continue;
                 }
                 result -= int((str[i]-'0')*j);
              }
              j *= 10; 
           }
           else 
           {
                max_or_min = 0;
                result = 0;
                j = 1;
           }
        }
        if (max_or_min != 0)
           return max_or_min;
        return result;
    }
};