LeetCode #1904 — MEDIUM

The Number of Full Rounds You Have Played

Move from brute-force thinking to an efficient approach using math strategy.

The Problem

Problem Statement

You are participating in an online chess tournament. There is a chess round that starts every 15 minutes. The first round of the day starts at 00:00, and after every 15 minutes, a new round starts.

For example, the second round starts at 00:15, the fourth round starts at 00:45, and the seventh round starts at 01:30.

You are given two strings loginTime and logoutTime where:

loginTime is the time you will login to the game, and
logoutTime is the time you will logout from the game.

If logoutTime is earlier than loginTime, this means you have played from loginTime to midnight and from midnight to logoutTime.

Return the number of full chess rounds you have played in the tournament.

Note: All the given times follow the 24-hour clock. That means the first round of the day starts at 00:00 and the last round of the day starts at 23:45.

Example 1:

Input: loginTime = "09:31", logoutTime = "10:14"
Output: 1
Explanation: You played one full round from 09:45 to 10:00.
You did not play the full round from 09:30 to 09:45 because you logged in at 09:31 after it began.
You did not play the full round from 10:00 to 10:15 because you logged out at 10:14 before it ended.

Example 2:

Input: loginTime = "21:30", logoutTime = "03:00"
Output: 22
Explanation: You played 10 full rounds from 21:30 to 00:00 and 12 full rounds from 00:00 to 03:00.
10 + 12 = 22.

Constraints:

loginTime and logoutTime are in the format hh:mm.
00 <= hh <= 23
00 <= mm <= 59
loginTime and logoutTime are not equal.

Step 01

Brute Force Baseline

Problem summary: You are participating in an online chess tournament. There is a chess round that starts every 15 minutes. The first round of the day starts at 00:00, and after every 15 minutes, a new round starts. For example, the second round starts at 00:15, the fourth round starts at 00:45, and the seventh round starts at 01:30. You are given two strings loginTime and logoutTime where: loginTime is the time you will login to the game, and logoutTime is the time you will logout from the game. If logoutTime is earlier than loginTime, this means you have played from loginTime to midnight and from midnight to logoutTime. Return the number of full chess rounds you have played in the tournament. Note: All the given times follow the 24-hour clock. That means the first round of the day starts at 00:00 and the last round of the day starts at 23:45.

Baseline thinking

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

Pattern signal: Math

Example 1

"09:31"
"10:14"

Example 2

"21:30"
"03:00"

Step 02

Core Insight

What unlocks the optimal approach

Consider the day as 48 hours instead of 24.
For each round check if you were playing.

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 #1904: The Number of Full Rounds You Have Played
class Solution {
    public int numberOfRounds(String loginTime, String logoutTime) {
        int a = f(loginTime), b = f(logoutTime);
        if (a > b) {
            b += 1440;
        }
        return Math.max(0, b / 15 - (a + 14) / 15);
    }

    private int f(String s) {
        int h = Integer.parseInt(s.substring(0, 2));
        int m = Integer.parseInt(s.substring(3, 5));
        return h * 60 + m;
    }
}

// Accepted solution for LeetCode #1904: The Number of Full Rounds You Have Played
func numberOfRounds(loginTime string, logoutTime string) int {
	f := func(s string) int {
		var h, m int
		fmt.Sscanf(s, "%d:%d", &h, &m)
		return h*60 + m
	}
	a, b := f(loginTime), f(logoutTime)
	if a > b {
		b += 1440
	}
	return max(0, b/15-(a+14)/15)
}

# Accepted solution for LeetCode #1904: The Number of Full Rounds You Have Played
class Solution:
    def numberOfRounds(self, loginTime: str, logoutTime: str) -> int:
        def f(s: str) -> int:
            return int(s[:2]) * 60 + int(s[3:])

        a, b = f(loginTime), f(logoutTime)
        if a > b:
            b += 1440
        a, b = (a + 14) // 15, b // 15
        return max(0, b - a)

// Accepted solution for LeetCode #1904: The Number of Full Rounds You Have Played
/**
 * [1904] The Number of Full Rounds You Have Played
 *
 * You are participating in an online chess tournament. There is a chess round that starts every 15 minutes. The first round of the day starts at 00:00, and after every 15 minutes, a new round starts.
 *
 * 	For example, the second round starts at 00:15, the fourth round starts at 00:45, and the seventh round starts at 01:30.
 *
 * You are given two strings loginTime and logoutTime where:
 *
 * 	loginTime is the time you will login to the game, and
 * 	logoutTime is the time you will logout from the game.
 *
 * If logoutTime is earlier than loginTime, this means you have played from loginTime to midnight and from midnight to logoutTime.
 * Return the number of full chess rounds you have played in the tournament.
 * Note: All the given times follow the 24-hour clock. That means the first round of the day starts at 00:00 and the last round of the day starts at 23:45.
 *  
 * Example 1:
 *
 * Input: loginTime = "09:31", logoutTime = "10:14"
 * Output: 1
 * Explanation: You played one full round from 09:45 to 10:00.
 * You did not play the full round from 09:30 to 09:45 because you logged in at 09:31 after it began.
 * You did not play the full round from 10:00 to 10:15 because you logged out at 10:14 before it ended.
 *
 * Example 2:
 *
 * Input: loginTime = "21:30", logoutTime = "03:00"
 * Output: 22
 * Explanation: You played 10 full rounds from 21:30 to 00:00 and 12 full rounds from 00:00 to 03:00.
 * 10 + 12 = 22.
 *
 *  
 * Constraints:
 *
 * 	loginTime and logoutTime are in the format hh:mm.
 * 	00 <= hh <= 23
 * 	00 <= mm <= 59
 * 	loginTime and logoutTime are not equal.
 *
 */
pub struct Solution {}

// problem: https://leetcode.com/problems/the-number-of-full-rounds-you-have-played/
// discuss: https://leetcode.com/problems/the-number-of-full-rounds-you-have-played/discuss/?currentPage=1&orderBy=most_votes&query=

// submission codes start here

impl Solution {
    pub fn number_of_rounds(login_time: String, logout_time: String) -> i32 {
        0
    }
}

// submission codes end

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    #[ignore]
    fn test_1904_example_1() {
        let login_time = "09:31".to_string();
        let logout_time = "10:14".to_string();

        let result = 1;

        assert_eq!(Solution::number_of_rounds(login_time, logout_time), result);
    }

    #[test]
    #[ignore]
    fn test_1904_example_2() {
        let login_time = "21:30".to_string();
        let logout_time = "03:00".to_string();

        let result = 22;

        assert_eq!(Solution::number_of_rounds(login_time, logout_time), result);
    }
}

// Accepted solution for LeetCode #1904: The Number of Full Rounds You Have Played
function numberOfRounds(startTime: string, finishTime: string): number {
    const f = (s: string): number => {
        const [h, m] = s.split(':').map(Number);
        return h * 60 + m;
    };
    let [a, b] = [f(startTime), f(finishTime)];
    if (a > b) {
        b += 1440;
    }
    return Math.max(0, Math.floor(b / 15) - Math.ceil(a / 15));
}

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.