String to Integer (atoi)

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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.

Requirements for atoi:

The function first discards as many whitespace characters as necessary until the first non-whitespace character is found. Then, starting from this character, takes an optional initial plus or minus sign followed by as many numerical digits as possible, and interprets them as a numerical value.

The string can contain additional characters after those that form the integral number, which are ignored and have no effect on the behavior of this function.

If the first sequence of non-whitespace characters in str is not a valid integral number, or if no such sequence exists because either str is empty or it contains only whitespace characters, no conversion is performed.

If no valid conversion could be performed, a zero value is returned. If the correct value is out of the range of representable values, INT_MAX (2147483647) or INT_MIN (-2147483648) is returned.

 

該題最好先考慮各種輸入情況，再動手寫代碼

輸入：

空串

125

12.36

546a

#4655

     45656

-0001

2147483647

-2147483648

9999999999999999

-99999999999999

 1 class Solution { 2 public: 3     int atoi(const char *str) { 4         if( !str ) return 0; 5         while( *str && *str == ‘ ‘ ) ++str; //尋找第一個非空白字元 6         bool minus = false; 7         if( *str == ‘-‘ ) { //判斷是否為負 8             minus = true; 9             ++str;10         }11         else if( *str == ‘+‘ ) ++str;   //或‘+‘12         long long ans = 0;  //為防止溢出，使用long long13         while( *str && isdigit(*str) ) {    //碰到非數字或結束符就結束14             ans = ans * 10 + (*str - ‘0‘);15             ++str;16         }17         ans = minus ? -ans : ans;   //還原真實值18         if( ans > INT_MAX ) return INT_MAX; //處理上溢19         else if( ans < INT_MIN ) return INT_MIN;    //處理下溢20         else return ans;21     }22 };

 

String to Integer (atoi)