String to Integer (atoi)

convert string to integer.

pay attention to the following problems:
the number might be positive/negative.
there might be spaces on the first/last
there might be other characters in the given string, and if the first char is a non numerical char, then we return 0
if it is overflow the integer range, please return the closest boundary.

class Solution {
    public int myAtoi(String str) {
        str = str.trim();
        if(str==null||str.length()==0) return 0;
        
        //char firstChar = str.charAt(0);
        int start = 0;//this indicates the first numerical char index of this string
        int sign = 1;
        long res = 0;//potentiially overflow preprocess

//initial start and sign
        if(str.charAt(0) == '+'){
            start = 1;
        } else if(str.charAt(0) == '-'){
            start = 1;
            sign = -1;
        }
        //iterate from start to the end
        for(int i = start;i<str.length();i++) {
            if(!Character.isDigit(str.charAt(i))){ //if find something that is not numerical, then we just break this
                break; 
            }
            //classic string to integer
            res = res * 10 + str.charAt(i) - '0';
            
            //check if overflow or not
            if(sign == 1 && res > Integer.MAX_VALUE) return Integer.MAX_VALUE;
            if(sign == -1 && res > Integer.MAX_VALUE) return Integer.MIN_VALUE;//至于这边为什么不能res>Integer.MAX_VALUE+1,因为Integer.MAX_VALUE+1了之后就变成Integer.MIN_VALUE了而如果res>Integer.MAX_VALUE的话 res>=abs(Integer.min)都会返回MIN_VALUE了
        }      
        
        return (int)res * sign; //return int
    }
}

the code above have the clearest mind.