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Problem 8: String to Integer (atoi)

Problem 8: String to Integer (atoi)

链接:https://leetcode.com/problems/string-to-integer-atoi/

难度等级:Medium(中等)

题干

Implement atoi which converts a string to an integer.

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.

题意

题意很简单,虽然写了很长一段,但是说白了只有一句话,就是把string转换成int。

样例

Example 1:

Input: "42"
Output: 42
Example 2:

Input: "   -42"
Output: -42
Explanation: The first non-whitespace character is '-', which is the minus sign.
             Then take as many numerical digits as possible, which gets 42.
Example 3:

Input: "4193 with words"
Output: 4193
Explanation: Conversion stops at digit '3' as the next character is not a numerical digit.
Example 4:

Input: "words and 987"
Output: 0
Explanation: The first non-whitespace character is 'w', which is not a numerical 
             digit or a +/- sign. Therefore no valid conversion could be performed.
Example 5:

Input: "-91283472332"
Output: -2147483648
Explanation: The number "-91283472332" is out of the range of a 32-bit signed integer.
             Thefore INT_MIN (−231) is returned.

注意

Only the space character ' ' is considered as whitespace character.

Assume we are dealing with an environment which could only store integers within the 32-bit signed integer range: [−231, 231 − 1]. If the numerical value is out of the range of representable values, INT_MAX (231 − 1) or INT_MIN (−231) is returned.

分析

这道题的字面难度很简单,一眼就能想到解法。但是真正实现起来却并不容易,尤其是想要一次性通过非常困难。并不是因为编码复杂,也不是思路困难,核心的问题只有一个,就是这道题的trick太多了。

题目没有对字符串当中的元素做任何限制,除了允许有空格之外,包括正负号以及字母都是允许的。而且数字前的空格是允许的,但是字母是不行的。合法数字后的字符都会被跳过。

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