What is the day of the year problem?

The problem asks to calculate the day number of the year based on a given Gregorian date formatted as YYYY-MM-DD.

How do you handle leap years in the day of the year problem?

Leap years are handled by checking if the year is divisible by 4, not divisible by 100 unless also divisible by 400. If true, February has 29 days.

What should be considered when parsing a date in the day of the year problem?

Consider extracting the year, month, and day, and correctly managing the leap year condition for February to avoid errors in calculations.

How does the month-day array help in solving the day of the year problem?

The month-day array allows for an efficient lookup of the number of days in each month, including adjustments for leap years, making the calculation faster and easier.

What are common pitfalls in solving the day of the year problem?

Common pitfalls include failing to handle leap years, incorrect array indexing for months, and missing edge cases like the first and last days of the year.

#1154

Easy

auto_awesome数学·string

LeetCode 题解工作台

一年中的第几天

给你一个字符串 date ，按 YYYY-MM-DD 格式表示一个现行公元纪年法日期。返回该日期是当年的第几天。示例 1：输入： date = "2019-01-09" 输出： 9 解释：给定日期是2019年的第九天。示例 2：输入： date = "2019-02-10" 输出： 4…

数学字符串

题目描述

给你一个字符串 date ，按 YYYY-MM-DD 格式表示一个现行公元纪年法日期。返回该日期是当年的第几天。

示例 1：

输入：date = "2019-01-09"
输出：9
解释：给定日期是2019年的第九天。

示例 2：

输入：date = "2019-02-10"
输出：41

提示：

date.length == 10
date[4] == date[7] == '-'，其他的 date[i] 都是数字
date 表示的范围从 1900 年 1 月 1 日至 2019 年 12 月 31 日

lightbulb

解题思路

方法一：直接计算

根据题意，给定的日期是公元纪年法的日期，因此可以直接计算出该日期是当年的第几天。

首先，根据给定的日期计算出年月日，分别为 $y$ , $m$ , $d$ 。

然后，根据公元纪年法的闰年规则，计算出当年二月份的天数，闰年的二月份有 $29$ 天，平年的二月份有 $28$ 天。

闰年的计算规则是：年份能被 $400$ 整除，或者年份能被 $4$ 整除且不能被 $100$ 整除。

最后，根据给定的日期计算出当年的第几天，即把前面每个月的天数累加起来，再加上当月的天数即可。

时间复杂度 $O(1)$ ，空间复杂度 $O(1)$ 。

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class Solution:
    def dayOfYear(self, date: str) -> int:
        y, m, d = (int(s) for s in date.split('-'))
        v = 29 if y % 400 == 0 or (y % 4 == 0 and y % 100) else 28
        days = [31, v, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]
        return sum(days[: m - 1]) + d

speed

复杂度分析

指标	值
时间	Depends on the final approach
空间	Depends on the final approach

psychology

面试官常问的追问

外企场景

question_mark
Look for candidates who correctly handle leap years using conditional checks.
question_mark
Evaluate understanding of how arrays can optimize counting days across months.
question_mark
Ensure that candidates can break down the problem into small, manageable steps.

warning

常见陷阱

外企场景

error
Forgetting to account for leap years, which can cause an off-by-one error in February.
error
Incorrectly indexing the month, as arrays in programming are often zero-indexed, but months are one-indexed in this problem.
error
Not handling edge cases, such as dates in December or the very start of the year.

swap_horiz

进阶变体

外企场景

arrow_right_alt
Handling different date formats, such as MM-DD-YYYY.
arrow_right_alt
Calculating the day number for a different range of years, not limited to 1900–2019.
arrow_right_alt
Optimizing the approach for larger datasets or multiple date calculations in one go.

help

常见问题

外企场景

继续练习

#1071 字符串的最大公因子 #1247 交换字符使得字符串相同 #1041 困于环中的机器人