LeetCode Online Judge 题目C# 练习 - String to Integer (atoi)

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.

public static int StringtoInteger(string str)
{
int i = 0;
int sign = 0;
int val = 0;

while (i < str.Length && ((str[i] >= '0' && str[i] <= '9') || str[i] == ' ' || str[i] == '-' || str[i] == '+'))
{
if ((val == 0 && sign == 0) && str[i] == ' ')
i++;
else if (str[i] == '-' && sign == 0)
{
sign = -1;
i++;
}
else if (str[i] == '+' && sign == 0)
{
sign = 1;
i++;
}
else if (str[i] >= '0' && str[i] <= '9')
{
//handle overflow, val * 10 + n > int.MaxValue
if (val > (int.MaxValue - (str[i] - '0')) / 10)
{
if (sign == 0 || sign == 1)
return int.MaxValue;
return int.MinValue;
}
val = val * 10 + str[i] - '0';
i++;
}
else
{
if (sign == 0)
return val;
return val * sign;
}
}
if (sign == 0)
return val;
return val * sign;
}

代码分析：

　　atoi, 最重要是讨论输入的validation。怎么样的输入才是合法输入，如果碰到不合法的字符，返回什么东西。而且还要处理一下overflow。

　　当然主要逻辑就是这一句了val = val * 10 + str[i] - '0';

　　我这里是按照LeetCode上的Test Case 写的， 迎合大牛的口味。但是面试的时候要跟面试官详细讨论。