8. String to Integer (atoi) Java Solutin

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 a const char * argument, please click the reload button  to reset your code definition.

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题意： 

实现 atoi,将字符串转化为整数。

注意考虑所有可能的情况。

算法分析： 

1. 
先去掉多余的空格字符，从第一个非空格字符开始。

2. 
按顺序读数字，如果出现下面三种情况则结束：

2.1 
异常字符出现（异常字符跳过不计）。

2.2 
数字越界（返回最接近的整数）。

2.3 
字符串结束

public class Solution {
    public int myAtoi(String str) {
        if(str == null) return 0;
        str = str.trim();
        if(str.length() == 0) return 0;
        Boolean isNex = false;
        int i = 0;
        if(str.charAt(0) == '+' || str.charAt(0) == '-'){
            i++;
            if(str.charAt(0) == '-')
                isNex = true;
        }
        long result = 0;//若定义为int，边界值将会溢出。
        for(;i<str.length();i++){
            if(str.charAt(i) > '9' || str.charAt(i) < '0')
                break;
            result = result*10 + str.charAt(i) - '0';
            if(result > Integer.MAX_VALUE && !isNex)
                return Integer.MAX_VALUE;
            if(-result < Integer.MIN_VALUE && isNex)
                return Integer.MIN_VALUE;
        }
        if(!isNex) return (int) result;
        else result = -result;
        return (int)result;
    }
}