LeetCode8. String to Integer (atoi)（C++/Python）

最新推荐文章于 2024-06-19 07:54:23 发布

原创最新推荐文章于 2024-06-19 07:54:23 发布 · 230 阅读

0 ·

CC 4.0 BY-SA版权

Leetcode 专栏收录该内容

110 篇文章

订阅专栏

本文介绍了一个将字符串转换为整数的atoi函数实现方法，详细解释了如何处理各种特殊情况，包括字符串前导空格、正负号、非数字字符以及超出32位整数范围的情况。

摘要生成于 C知道，由 DeepSeek-R1 满血版支持，前往体验 >

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.

Note:

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.

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.

题意要自己写出atoi，题目不难理解，如果待转化的数比超过int的范围，需要返回INT_MAX或者INT_MIN，所以这里先用long long int 的类型，但是有几个特殊情况要留意：

1、待转化的数字字符串超过long long int的范围，所以我们要限定一下位数，超过11位肯定超过int的范围，就不用继续遍历了；

2、待转化的数字字符串前面有很多0，由于在1中我们限定了字符串的长度，为了防止把符合的字符串漏掉，所以要先去除0；

C++

class Solution {
public:
    int myAtoi(string str) {
        long long int result = 0;
        int i = 0, len = str.length(), flag = 1, cnt = 0;
        while(i < len && str[i] == ' ')
            ++ i;
        if(i == str.length() || (!isdigit(str[i]) && str[i] != '-' && str[i] != '+'))
            return 0;
        if(str[i] == '+' || str[i] == '-')
            flag = (str[i++] == '-') ? -1 : 1;
        while(i < len && str[i] == '0')
            ++ i;
        while(i < len && isdigit(str[i]) && cnt++ < 11)
            result = result * 10 + (str[i++] - '0');
        result = flag * result;
        return result > INT_MAX ? INT_MAX : result < INT_MIN ? INT_MIN : (int)result;
    }
};

Python

class Solution(object):
    def myAtoi(self, str):
        """
        :type str: str
        :rtype: int
        """
        result, flag, i, cnt, Len = 0, 1,0, 0, len(str)
        INT_MAX, INT_MIN = 2**31-1, -2**31
        while i < Len and str[i] == ' ':
            i += 1
        if i == Len or not (str[i] == '+' or str[i] == '-' or str[i].isdigit()):
            return 0
        if str[i] == '+' or str[i] == '-':
            flag = -1 if str[i] == '-' else 1
            i += 1
        while i < Len and str[i] == '0':
            i += 1
        while i < Len and str[i].isdigit() and cnt < 11:
            result = result * 10 + (ord(str[i]) - ord('0'))
            i += 1
            cnt += 1
        result = result * flag
        return int(result) if result <= INT_MAX and result >= INT_MIN else (INT_MAX if result > INT_MAX else INT_MIN)