[LeetCode] String to Integer

Thought:
Skip whitespace -> Check Sign -> Check Digit -> Check Overflow -> Result
When dealing with overflow, we could write  neither 
result * 10 > Integer.MAX_VALUE - temp nor  
result * 10 + temp > Integer.MAX_VALUE 
Because both of their left side might overflows which will lead to the wrong answer.

Code:
public class Solution {
    public int atoi(String str) {
      
        int i = 0, result = 0, sign = 1;
        char c = ' ';
      
        while (i  < str.length()) {
            c = str.charAt(i);
            if (Character.isWhitespace(c)) {
                i++;
            }else if (c == '+' || c == '-' ) {
                i++;
                if( c == '-' ) sign = -1;
                break;
            }else if(Character.isDigit(c)){
                break;
            }else{
                return 0;          
            }
        }      
        while (i < str.length()) {
            c = str.charAt(i);
            if (!Character.isDigit(c)){
                break;
            }else{
                int temp = Character.digit(c, 10);
                if( result > (Integer.MAX_VALUE - temp)/10 ){
                    return sign < 0? Integer.MIN_VALUE: Integer.MAX_VALUE;
                }else{
                    result = result * 10 + temp;
                }
            }
            i++;
        }
      
        return sign * result;
    }
}