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class Solution {//note://1. NULL//2. sign//3. prefix ' '//4. out of range, to avoid this we use long longpublic:	int atoi(const char *str) {		// Start typing your C/C++ solution below		// DO NOT write int main() function		if(str == NULL) return 0;//NULL		while (*str == ' ')//prefix ' ' 			str++;		int sign = 1;		if(*str == '-')//sign '+' or '-'		{			sign = -1;			str++;		}		else if(*str == '+')			str++;		long long ans = 0;		while (*str >= '0' && *str <= '9')		{			ans = ans*10+*str-'0';			if(ans > INT_MAX)//out of range				return sign < 0 ? INT_MIN : INT_MAX;			str++;		}		ans *= sign;		return (int)ans;	}};

second time

class Solution {public:    int atoi(const char *str) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        if(str == NULL) return 0;                bool sign = true;                while (*str == ' ')//prefix ' '               str++;                      if(*str == '+' || *str == '-')        {            if(*str == '+') sign = true;            else sign = false;            str++;        }                 long long ans = 0;        while(*str >= '0' && *str <= '9')        {            ans = ans*10+(*str-'0');            if(ans > INT_MAX) return sign == true ? INT_MAX : INT_MIN;            str++;        }                if(sign == false) return (int)(-ans);        else return (int)ans;    }};

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