LeetCode #1154 — EASY

Day of the Year

Build confidence with an intuition-first walkthrough focused on math fundamentals.

The Problem

Problem Statement

Given a string date representing a Gregorian calendar date formatted as YYYY-MM-DD, return the day number of the year.

Example 1:

Input: date = "2019-01-09"
Output: 9
Explanation: Given date is the 9th day of the year in 2019.

Example 2:

Input: date = "2019-02-10"
Output: 41

Constraints:

date.length == 10
date[4] == date[7] == '-', and all other date[i]'s are digits
date represents a calendar date between Jan 1^st, 1900 and Dec 31^st, 2019.

Step 01

Brute Force Baseline

Problem summary: Given a string date representing a Gregorian calendar date formatted as YYYY-MM-DD, return the day number of the year.

Baseline thinking

Start with the most direct exhaustive search. That gives a correctness anchor before optimizing.

Pattern signal: Math

Example 1

"2019-01-09"

Example 2

"2019-02-10"

Step 02

Core Insight

What unlocks the optimal approach

Have a integer array of how many days there are per month. February gets one extra day if its a leap year. Then, we can manually count the ordinal as day + (number of days in months before this one).

Interview move: turn each hint into an invariant you can check after every iteration/recursion step.

Step 03

Algorithm Walkthrough

Iteration Checklist

Define state (indices, window, stack, map, DP cell, or recursion frame).
Apply one transition step and update the invariant.
Record answer candidate when condition is met.
Continue until all input is consumed.

Use the first example testcase as your mental trace to verify each transition.

Step 04

Edge Cases

Minimum Input

Single element / shortest valid input

Validate boundary behavior before entering the main loop or recursion.

Duplicates & Repeats

Repeated values / repeated states

Decide whether duplicates should be merged, skipped, or counted explicitly.

Extreme Constraints

Upper-end input sizes

Re-check complexity target against constraints to avoid time-limit issues.

Invalid / Corner Shape

Empty collections, zeros, or disconnected structures

Handle special-case structure before the core algorithm path.

Step 05

Full Annotated Code

Source-backed implementations are provided below for direct study and interview prep.

// Accepted solution for LeetCode #1154: Day of the Year
class Solution {
    public int dayOfYear(String date) {
        int y = Integer.parseInt(date.substring(0, 4));
        int m = Integer.parseInt(date.substring(5, 7));
        int d = Integer.parseInt(date.substring(8));
        int v = y % 400 == 0 || (y % 4 == 0 && y % 100 != 0) ? 29 : 28;
        int[] days = {31, v, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
        int ans = d;
        for (int i = 0; i < m - 1; ++i) {
            ans += days[i];
        }
        return ans;
    }
}

// Accepted solution for LeetCode #1154: Day of the Year
func dayOfYear(date string) (ans int) {
	var y, m, d int
	fmt.Sscanf(date, "%d-%d-%d", &y, &m, &d)
	days := []int{31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}
	if y%400 == 0 || (y%4 == 0 && y%100 != 0) {
		days[1] = 29
	}
	ans += d
	for _, v := range days[:m-1] {
		ans += v
	}
	return
}

# Accepted solution for LeetCode #1154: Day of the Year
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

// Accepted solution for LeetCode #1154: Day of the Year
struct Solution;

impl Solution {
    fn day_of_year(date: String) -> i32 {
        let a: Vec<&str> = date.split_terminator('-').collect();
        let year = a[0].parse::<usize>().unwrap();
        let month = a[1].parse::<usize>().unwrap();
        let day = a[2].parse::<usize>().unwrap();
        let mut days: Vec<usize> = vec![31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
        let mut sum = 0;
        if year % 4 == 0 && (year % 100 != 0 || year % 400 == 0) {
            days[1] += 1;
        }
        for i in 0..month - 1 {
            sum += days[i];
        }
        sum += day;
        sum as i32
    }
}

#[test]
fn test() {
    assert_eq!(Solution::day_of_year("2019-01-09".to_string()), 9);
    assert_eq!(Solution::day_of_year("2019-02-10".to_string()), 41);
    assert_eq!(Solution::day_of_year("2003-03-01".to_string()), 60);
    assert_eq!(Solution::day_of_year("2004-03-01".to_string()), 61);
}

// Accepted solution for LeetCode #1154: Day of the Year
function dayOfYear(date: string): number {
    const y = +date.slice(0, 4);
    const m = +date.slice(5, 7);
    const d = +date.slice(8);
    const v = y % 400 == 0 || (y % 4 == 0 && y % 100) ? 29 : 28;
    const days = [31, v, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
    return days.slice(0, m - 1).reduce((a, b) => a + b, d);
}

Step 06

Interactive Study Demo

Use this to step through a reusable interview workflow for this problem.

Custom checkpoints (one per line)

Press Step or Run All to begin.

Step 07

Complexity Analysis

Time

O(1)

Space

O(1)

Approach Breakdown

ITERATIVE

O(n) time

O(1) space

Simulate the process step by step — multiply n times, check each number up to n, or iterate through all possibilities. Each step is O(1), but doing it n times gives O(n). No extra space needed since we just track running state.

MATH INSIGHT

O(log n) time

O(1) space

Math problems often have a closed-form or O(log n) solution hidden behind an O(n) simulation. Modular arithmetic, fast exponentiation (repeated squaring), GCD (Euclidean algorithm), and number theory properties can dramatically reduce complexity.

Shortcut: Look for mathematical properties that eliminate iteration. Repeated squaring → O(log n). Modular arithmetic avoids overflow.

Coach Notes

Common Mistakes

Review these before coding to avoid predictable interview regressions.

Overflow in intermediate arithmetic

Wrong move: Temporary multiplications exceed integer bounds.

Usually fails on: Large inputs wrap around unexpectedly.

Fix: Use wider types, modular arithmetic, or rearranged operations.