String to Integer (atoi)

题目描述：

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.

Update (2015-02-10):
The signature of the C++ function had been updated. If you still see your function signature accepts aconst char * argument, please click the reload button to reset your code definition.

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.

这个题没有难度，就是要考虑一些情况。如果在面试的时候一定要将不确定怎么处理的情况告诉提出来，例如：

会不会出现非数字，会不会出现空格，开头会不会出现空格，中间出现空格怎么处理，超过最大值和最小值怎么处理。

我中途wrong answer碰到的出错的用例：

"+-2"，"   010"，"   +004500"，"-0012a42"，"2147483648"，"9223372036854775807".也算是惨的不行

代码如下：

public class Solution {    public int myAtoi(String str){        int startindex=0,positive=1;        long result=0;        if(str.length()<1)        return 0;        while(str.charAt(startindex)==' ')        startindex++;        if(str.charAt(startindex)=='-')        positive=-1;        if(str.charAt(startindex)=='-'||str.charAt(startindex)=='+')        startindex++;        while(startindex<str.length()){        if((str.charAt(startindex)<'0'||str.charAt(startindex)>'9'))        break;        else if(str.charAt(startindex)>='0'&&str.charAt(startindex)<='9'){        result=result*10+str.charAt(startindex)-'0';        if(result>Integer.MAX_VALUE)        break;        }        startindex++;        }        result*=positive;        return (int)(result>Integer.MAX_VALUE?Integer.MAX_VALUE:result<Integer.MIN_VALUE?Integer.MIN_VALUE:result);    }}