【leetcode】String to Integer (atoi)

题目：

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.
spoilers alert... click to show requirements
for atoi.

解析：将字符串转化成整数，有很多需要注意的地方：

1. 将字符串开头的空格删去

2. 整数可能是正或者负

3. 注意结果可能越界，如果大于最大值2147483647则最后返回最大值，如果小于最小值-2147483648则最后返回最小值。

4. leetcode的case中没有考虑输入空指针，非数字的情况（题目没有表明异常输入该返回什么）

class Solution {
public:
    int atoi(const char *str) {
        long long result = 0;
        bool ifPositive = true;
        while(*str==' '){
            str++;
        }
        if(*str=='+'||*str=='-'){
            ifPositive = *str=='+'? true: false;
            str++;
        }
        while(*str<='9' && *str>='0'){
            result = result * 10 + *str - '0';
            str++;
        }
        if(ifPositive && result > 2147483647){
            result = 2147483647;
        }else if(!ifPositive){
            result = -result;
            if(result < -2147483648){
                result = -2147483648;
            }
        }
        return result;
    }
};