问题描写叙述：

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.

思路：

实现字符串转换为int型整数并不难，仅仅是有好多中输入情况须要考虑，easy造成遗漏。

首先看一下c++ 中atoi的定义：

-----------------------------------

atoi

int atoi (const char * str);

Return Value

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能够看出须要注意的有下面几点：

1. 去掉前面的空格
2. 注意開始时候的符号 + -
3. 忽略数字后面的杂七杂八字符
4. 考虑溢出的情况。

   
   

考虑了上面几方面。代码就能够AC了。

代码：

public class String_To_Integer { //java
	public static int atoi(String str) {
		int haveMinus = 1;
		long result = 0;  
		
		if(str == null || str.trim().isEmpty())
			return 0;
		
		//chech + - 
		str = str.trim();
		if(str.charAt(0) == ‘-‘){
			str = str.substring(1);
			haveMinus = -1;
		}else if(str.charAt(0) == ‘+‘)
			str = str.substring(1);
		
		//check num
		for(int i = 0;i < str.length() && str.charAt(i) >= ‘0‘ && str.charAt(i) <= ‘9‘; i++){
			result = result*10 + (str.charAt(i)- ‘0‘);
		}
		
		//deal overflow
		if(result > 2147483647 && haveMinus == 1)
			return 2147483647;
		if(result > 2147483647 && haveMinus == -1)
			return -2147483648;
		
		return haveMinus*(int)result;
    }
	
	public static void main(String [] args){
		System.out.println(String_To_Integer.atoi("2147483648"));
	}
}