67:把字符串转换成整数

class Solution {
public:
    int StrToInt(string str) {
        int start=0,res=0;
        if(str[0]=='-'||str[0]=='+')start++;
        for(int i=start;i<str.length();i++)
        {
            if(!(str[i]>='0'&&str[i]<='9'))return 0;
            res=res*10+str[i]-'0';
        }
        if(str[0]=='-')res=-res;
        return res;
    }
};