leetcode Question 103: String to Integer (atoi)

String to Integer (atoi)

Implement atoi to convert a string to an integer.

Hint: Carefully consider all possible input cases. If you want a challenge, please do not see below and ask yourself what are the possible input cases.

Notes: It is intended for this problem to be specified vaguely (ie, no given input specs). You are responsible to gather all the input requirements up front.

Analysis:
As there is no need to consider float number, what we need concern here is
(1) "+" and "-"
(2) The boundary INT_MAX and INT_MIN
(3) Eliminate the spaces before.
(4) Meet non-digit after digit then return.

Code(Updated 201309):

class Solution {
public:
    int atoi(const char *str) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if (!str){return 0;}
        int i=0;
        bool pos=true;
        int res=0;
        while (str[i]==' '){ i++;}
        if (str[i]=='+'){ pos=true;i++;}
        if (str[i]=='-'){ pos = false;i++;}
        if (!isdigit(str[i])){return 0;}
        while (isdigit(str[i])){
           if (pos && res>INT_MAX/10){return INT_MAX;}
           if (pos && res==INT_MAX/10 && int(str[i]-'0')>=7){return INT_MAX;}
           if (!pos && -res<INT_MIN/10){return INT_MIN;}
           if (!pos && -res==INT_MIN/10 && int(str[i]-'0')>=8){return INT_MIN;}
           res = res*10 + int(str[i]-'0');
           i++;
        }
            
        if (pos){return res;}
        else{return -res;}    
           
    }
};