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.

spoilers alert... click to show requirements for atoi.

Solution:
The key for this question is the corner cases:
1. space at the beginning of string.
2. sign bit before numbers.
3. overflow, since there is no string length limit.

public class Solution {
    public int atoi(String str) {
        int len = str.length();
        if(len == 0) return 0;
        int i = 0;
        boolean sign =  true;
        
        while(str.charAt(i) == ' ') i++;
        
        if(str.charAt(i) == '+' || str.charAt(i) == '-'){
            if(str.charAt(i) == '-'){
                sign = false;   
            }
            i++;
        }
        int ret = 0;
        for(; i < len; i++){
            int tmp = str.charAt(i) - '0';
            if(tmp < 0 || tmp > 9) break;
            
            //ret * 10 + tmp should be smaller than MAX_VALUE;Otherwise it may overflow
            if(ret > Integer.MAX_VALUE / 10 || 
               (ret == Integer.MAX_VALUE / 10 && tmp > Integer.MAX_VALUE % 10)){   
                return sign == true ? Integer.MAX_VALUE : Integer.MIN_VALUE;
            }
            
            ret = ret * 10 + tmp;
        }
        return sign == true ? ret : -ret;
    }
}