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.

题目含义：实现atoi功能

 1     public int myAtoi(String str) {
 2     if (str == null || str.length() == 0)
 3         return 0;//
 4     str = str.trim();
 5     char firstChar = str.charAt(0);
 6     int sign = 1, start = 0, len = str.length();
 7     long sum = 0;
 8     if (firstChar == '+') {
 9         sign = 1;
10         start++;
11     } else if (firstChar == '-') {
12         sign = -1;
13         start++;
14     }
15     for (int i = start; i < len; i++) {
16         if (!Character.isDigit(str.charAt(i)))
17             return (int) sum * sign;
18         sum = sum * 10 + str.charAt(i) - '0';
19         if (sign == 1 && sum > Integer.MAX_VALUE)
20             return Integer.MAX_VALUE;
21         if (sign == -1 && (-1) * sum < Integer.MIN_VALUE)
22             return Integer.MIN_VALUE;
23     }
24 
25     return (int) sum * sign;        
26     }