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.

程序代码：

int myAtoi(char* str) {
    char *p = str;
    int flag = 1;
    while(*p==' ')
        p++;
    if(*p=='-'||*p=='+')
    {
        flag = 1-2*(*p=='-');
        p++;
    }
    int sum = 0;
    while(*p>='0'&&*p<='9')
    {
        if(sum>INT_MAX/10||(sum==INT_MAX/10&&*p-'0'>7))
        {
            if(flag==-1)    return INT_MIN;
            else        return INT_MAX;
        }
        sum = sum *10 +*p-'0';
        p++;
    }
    return sum*flag;
}