参考资料：

[1]题目：https://leetcode-cn.com/problems/string-to-integer-atoi/submissions/
[2]编程之法：C++面试和算法心得

注意：

//范围超限的问题

class Solution {
public:
    int myAtoi(string str) {
        
    
        const char* pStr = str.data();
        
        const int INTMAX = 2147483647;
        const int INTMIN = -2147483648;
        
        if(pStr == nullptr)
            return 0;
        
        while(isspace(*pStr))
            pStr++;
        bool bFlag = true;
        
        if(*pStr =='+')
        {
            bFlag = true;
            pStr++;
        }
        else if(*pStr == '-')
        {
            bFlag = false;
            pStr++;
        }   
        int n=0;
        while(pStr!=nullptr)
        {
            if(isdigit(*pStr))
            {
                int c = *pStr-'0';
                
                //判断n是否超出范围
                if((bFlag == true) &&((n>INTMAX/10) || ((n==INTMAX/10) && (c>INTMAX%10))))
                {
                    return INTMAX;
                }
                else if((bFlag == false) &&((n>(unsigned)INTMIN/10 )|| ((n==(unsigned)INTMIN/10) && (c>(unsigned)INTMIN%10))))
                {
                    return INTMIN;
                }
                //判断n是否超出范围
                n=10*n+c;
            }
            else
            {
                break;
            }
            pStr++;
        }
        
        if(bFlag == true)
            return n;
        else
            return -n;
    }
};