8. 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.

思路：四种情况一步一步来，最美的是int digit=s.charAt(i)-'0'

public class Solution {
    public int myAtoi(String str) {
    int index=0, sign=1, result=0;
    //1. Empty string
    if(str.length()==0) return 0;

    //2. Remove space
    while(str.charAt(index)==' '&&index<str.length())
        index++;

    //3. sign
    if(str.charAt(index)=='+'||str.charAt(index)=='-'){
        sign = str.charAt(index)=='+'? 1:-1;
        index++;        
    }
    if (index==str.length())
        return 0;
    if(str.charAt(index)=='+'||str.charAt(index)=='-')
        return 0;

    //4. convert number
    while(index<str.length()){
        int digit = str.charAt(index)-'0';
        if(digit < 0 || digit > 9) break;

        if(Integer.MAX_VALUE/10 < result || Integer.MAX_VALUE/10 == result && Integer.MAX_VALUE %10 < digit){
            return sign == 1 ? Integer.MAX_VALUE : Integer.MIN_VALUE;
        }
        int newResult = result*10+digit;
        result = newResult;
        index++;
    }

    return result*sign;
    }
}