49.把字符串转换成整数

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class Solution {
public:
    int StrToInt(string str) {
        int len = str.size();
        if(len == 0||str.empty()) return 0;
        int sum =0;
        char zero = 0+'0';
        char nine = 9+'0';
        bool flag =true;
        bool flg =false;
        if(str[0]=='-') flag = false;
        if(str[0]=='+') flg =true;
        if(flag){
            if(!flg){    
                for(int i=0;i<len;i++){
                if(str[i]>=zero && str[i]<=nine){
                    sum = sum*10 + (str[i] - '0');

                }
                else return 0;
            }
              return sum;}
            else{
                for(int i=1;i<len;i++){
                if(str[i]>=zero && str[i]<=nine){
                    sum = sum*10 + (str[i] - '0');
                }
                else return 0;
            }
              return sum;
            }

        }

        if(!flag){
            for(int i=1;i<len;i++){
                if(str[i]>=zero && str[i]<=nine){
                    sum = sum*10 + (str[i] - '0');
                }
                else return 0;
            }
              return -sum;
        }
        return 0;
    }

};