8. String to Integer (atoi) (重要)

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.

注意5点。

1.空字符串

2.字符串前的空格

3.+-号

4.非法字符（这里返回非法字符前的有效位）

5.溢出（这里如果大于最大，返回最大；小于最小返回最小）

class Solution {
public:
	int myAtoi(string str) {
		int len = str.size();
		if (len == 0) return 0;

		long long sum = 0;
		int i = 0;
		while (str[i] == ' ') i++;

		int minus = 1;
		if (str[i] == '-'){
			minus = -1;
			i++;
		}
		else if (str[i] == '+'){
			i++;
		}

		for (; i < len; i++){
			if (str[i]<'0' || str[i]>'9'){
				break;
			}
			sum = sum*10 + (str[i] - '0')*minus;
			if (sum>INT_MAX){
				return INT_MAX;
			}
			else if (sum < INT_MIN){
				return INT_MIN;
			}
		}
		return sum;
	}
};