LeetCode---(8)String to Integer (atoi)

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.

首先，字符串开始的空白字符要跳过，并且需要记录首字母是‘+’‘-’或者直接就是数字，以此来判断正负数。

其次，对于转换结果，我们知道整型数据的范围是INT_MIN（-2147482648）到INT_MAX（2147483647），超出范围则返回最大与最小值。所以我们可以开始用long long类型的变量存储结果；

class Solution {public:    int myAtoi(string str) {        if(str.size()==0)            return 0;        long long res=0;        int flag=1;        int i=0;        while(str[i]==' ')            i++;        if(str[i]=='+')            i++;        else if(str[i]=='-'){            flag=-1;            i++;        }        for(int j=i;j<str.size();j++)        {            if(str[j]<='9'&&str[j]>='0')            {                    res=res*10+(str[j]-'0');                if(res>INT_MAX)                    return flag>0?INT_MAX:INT_MIN;            }            else                break;        }        return res*flag;    }};