注意if 的顺序很重要，这样才能做到一次遍历就完事

#include <stdlib.h>
#include <math.h>
#include <string>
class Solution {
public:
    int strToInt(string str) {
        int i=0;
        int minus=1;
        long numbers=0;
        int size =str.size();
        int flag=0;
        int PN =0;
        while (i<size)
        {
           if (str[i]=='-') 
           {
               minus=-1;
               if (flag==1) break;
               flag=0;

           }
           else if(str[i]=='+') 
           {
               minus=1;
               if (flag==1) break;
               flag=0;

           }
           else if (str[i]==' ')
           {
               if (flag==1) break;
               flag=0;
           }
           else if(str[i]<'0'|| str[i]>'9' )
           {
                //只要有字母都会break
               break;
           }
           else
           {
            numbers = numbers*10 +str[i]-'0';
            flag=1;
            }
           i++;

        }
        numbers*=minus;
        if(numbers>INT_MAX) return INT_MAX;
        if(numbers<INT_MIN) return INT_MIN;
        return numbers; 

    }
};