https://leetcode.com/problems/string-to-integer-atoi/

class Solution {
public:
    const int _MAX=0x7fffffff,_MIN=0x80000000;
    int myAtoi(string str) {
        long long int val=0;
        int i=0,flag=1;
        while(str[i]==' ') ++i;
        if(str[i]=='-') flag=-1,++i;
        else if(str[i]=='+') ++i;
        while(i<str.size() && str[i]>='0' && str[i]<='9'){
            val=val*10+str[i]-'0';
            if(flag*val>=_MAX) return _MAX;
            if(flag*val<=_MIN) return _MIN;
            ++i;
        }
        return flag*val;
    }
};