leetcode String to Integer 008

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.

#include <iostream>
#include <vector>
#include <algorithm>
#include <string>
using namespace std;

class Solution {
public:
	int myAtoi(string str) {
		int i = 0;
		int flag = 1;
		const int MAXX = 0x7fffffff;

		for (i = 0; i < str.length(); ++i)
		{
			if(str[i] == '-') {
				flag = -1;
				i = i+1;
				break;
			}
			if(str[i]=='+') {
				flag = 1;
				i = i+1;
				break;
			}
			if(str[i] != ' ') break;
		}
		
		int sum = 0, overflow=0;

		for (; i < str.length(); ++i)
		{
			if(str[i] >= '0' && str[i] <= '9') {
				if(MAXX/10 < sum) {
					sum = MAXX;
					overflow = 1;
					break;
				}

				sum = sum*10+str[i]-'0';

				if(sum < 0) {
					sum = MAXX;
					overflow = 1;
					break;
				}

			} else {
				break;
			}
		}

		if(overflow) {
			if(flag==1) return sum;
			else if(flag==-1) return sum+1;
		} else {
			return flag*sum;
		}
	}
};