leetcode: String to Integer (atoi)

Problem Description:

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.

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.</p>

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.

Solution and Precautions:

Following the hint, it is not too difficult, two points are worth noting:

(1) The difference between INT_MAX (2147483647) and INT_MIN (-2147483648), click HERE two get further idea about it.

(2) The way to determine overflow is tricky, one should use the exact value of 2147483647 to check, one cannot use something like check whether the sign bit changes or not, or check if the new result get smaller to determine if the over flow occurs.

Written on April 21, 2013