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.

列举需要注意的集中情况

1、去空格 trim()

2、首字母为“+”或者“-”，则要记录下来。设一个flag变量，用于存储符号位

3、数字前出现了非数字字符，则返回0

4、在数字中出现了非数字字符，则返回前面的一些数字字符组成的数，后面的直接舍弃

5、存在数字越界的情况，所以要对最后得到的数字进行判断

public static int atoi(String str){        if(str == null || str.length() == 0)        {            return 0;        }        str = str.trim();        int flag = 1;        int start = 0;        long res = 0;        if(str.charAt(0)=='+'){            flag = 1;            start++;        }else if(str.charAt(0)=='-'){            flag = -1;            start++;        }        for(int i = start;i
     
       Integer.MAX_VALUE)            return Integer.MAX_VALUE;        if (flag == -1 && (-1) * res < Integer.MIN_VALUE)            return Integer.MIN_VALUE;                return (int)(flag * res);            }