LeetCode 题解（18）: String to Integer (atoi)

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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.

题解：

需要注意各种特殊输入，比如字符串开始有0或有空格，比如加减号，比如有字母出现，超出int的表示范围时（INT_MAX）需要根据符号返回对应的值（INT_MAX或INT_MIN）。isdigit()函数很省事，可以记住。

class Solution {
public:int atoi(const char *str) {long long result = 0;assert(str != NULL);bool positive = true;while(*str == '0' || *str == ' '){str++;}if(*str == '-'){positive = false;str++;}else if(*str == '+'){positive = true;str++;}while(isdigit(*str)){if(*str - '0' >= 0 && *str - '9' <= 0){result = result * 10 + (*str - '0');if(result > INT_MAX && positive)return INT_MAX;else if(result > INT_MAX && !positive)return INT_MIN;str++;}}if(positive)return result;elsereturn (0 - result);}
};

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