题目：
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.
Requirements for atoi:
The function first discards as many whitespace characters as
necessary until the first non-whitespace character is found. Then,
starting from this character, takes an optional initial plus or minus
sign followed by as many numerical digits as possible, and interprets
them as a numerical value.
The string can contain additional characters after those that form
the integral number, which are ignored and have no effect on the
behavior of this function.
If the first sequence of non-whitespace characters in str is not a
valid integral number, or if no such sequence exists because either str
is empty or it contains only whitespace characters, no conversion is
performed.
If no valid conversion could be performed, a zero value is returned.
If the correct value is out of the range of representable values,
INT_MAX (2147483647) or INT_MIN (-2147483648) is returned.
题解：
这题主要就是考虑一下corner case。
越界问题？
正负号问题？
空格问题？
精度问题？
代码如下：
1 public int atoi(String str) {
2 if (str == null || str.length() < 1)
3 return 0;
4
5 // trim white spaces
6 str = str.trim();
7
8 char flag = ‘+‘;
9
10 // check negative or positive
11 int i = 0;
12 if (str.charAt(0) == ‘-‘) {
13 flag = ‘-‘;
14 i++;
15 } else if (str.charAt(0) == ‘+‘) {
16 i++;
17 }
18 // use double to store result
19 double result = 0;
20
21 // calculate value
22 while (str.length() > i && str.charAt(i) >= ‘0‘ && str.charAt(i) <= ‘9‘) {
23 result = result * 10 + (str.charAt(i) - ‘0‘);
24 i++;
25 }
26
27 if (flag == ‘-‘)
28 result = -result;
29
30 // handle max and min
31 if (result > Integer.MAX_VALUE)
32 return Integer.MAX_VALUE;
33
34 if (result < Integer.MIN_VALUE)
35 return Integer.MIN_VALUE;
36
37 return (int) result;
38 }
Reference： http://www.programcreek.com/2012/12/leetcode-string-to-integer-atoi/
String to Integer (atoi) leetcode java