LeetCode #3586 — MEDIUM

Find COVID Recovery Patients

Move from brute-force thinking to an efficient approach using core interview patterns strategy.

Solve on LeetCode
The Problem

Problem Statement

Table: patients

+-------------+---------+
| Column Name | Type    |
+-------------+---------+
| patient_id  | int     |
| patient_name| varchar |
| age         | int     |
+-------------+---------+
patient_id is the unique identifier for this table.
Each row contains information about a patient.

Table: covid_tests

+-------------+---------+
| Column Name | Type    |
+-------------+---------+
| test_id     | int     |
| patient_id  | int     |
| test_date   | date    |
| result      | varchar |
+-------------+---------+
test_id is the unique identifier for this table.
Each row represents a COVID test result. The result can be Positive, Negative, or Inconclusive.

Write a solution to find patients who have recovered from COVID - patients who tested positive but later tested negative.

  • A patient is considered recovered if they have at least one Positive test followed by at least one Negative test on a later date
  • Calculate the recovery time in days as the difference between the first positive test and the first negative test after that positive test
  • Only include patients who have both positive and negative test results

Return the result table ordered by recovery_time in ascending order, then by patient_name in ascending order.

The result format is in the following example.

Example:

Input:

patients table:

+------------+--------------+-----+
| patient_id | patient_name | age |
+------------+--------------+-----+
| 1          | Alice Smith  | 28  |
| 2          | Bob Johnson  | 35  |
| 3          | Carol Davis  | 42  |
| 4          | David Wilson | 31  |
| 5          | Emma Brown   | 29  |
+------------+--------------+-----+

covid_tests table:

+---------+------------+------------+--------------+
| test_id | patient_id | test_date  | result       |
+---------+------------+------------+--------------+
| 1       | 1          | 2023-01-15 | Positive     |
| 2       | 1          | 2023-01-25 | Negative     |
| 3       | 2          | 2023-02-01 | Positive     |
| 4       | 2          | 2023-02-05 | Inconclusive |
| 5       | 2          | 2023-02-12 | Negative     |
| 6       | 3          | 2023-01-20 | Negative     |
| 7       | 3          | 2023-02-10 | Positive     |
| 8       | 3          | 2023-02-20 | Negative     |
| 9       | 4          | 2023-01-10 | Positive     |
| 10      | 4          | 2023-01-18 | Positive     |
| 11      | 5          | 2023-02-15 | Negative     |
| 12      | 5          | 2023-02-20 | Negative     |
+---------+------------+------------+--------------+

Output:

+------------+--------------+-----+---------------+
| patient_id | patient_name | age | recovery_time |
+------------+--------------+-----+---------------+
| 1          | Alice Smith  | 28  | 10            |
| 3          | Carol Davis  | 42  | 10            |
| 2          | Bob Johnson  | 35  | 11            |
+------------+--------------+-----+---------------+

Explanation:

  • Alice Smith (patient_id = 1):
    • First positive test: 2023-01-15
    • First negative test after positive: 2023-01-25
    • Recovery time: 25 - 15 = 10 days
  • Bob Johnson (patient_id = 2):
    • First positive test: 2023-02-01
    • Inconclusive test on 2023-02-05 (ignored for recovery calculation)
    • First negative test after positive: 2023-02-12
    • Recovery time: 12 - 1 = 11 days
  • Carol Davis (patient_id = 3):
    • Had negative test on 2023-01-20 (before positive test)
    • First positive test: 2023-02-10
    • First negative test after positive: 2023-02-20
    • Recovery time: 20 - 10 = 10 days
  • Patients not included:
    • David Wilson (patient_id = 4): Only has positive tests, no negative test after positive
    • Emma Brown (patient_id = 5): Only has negative tests, never tested positive

Output table is ordered by recovery_time in ascending order, and then by patient_name in ascending order.

Roadmap

  1. Brute Force Baseline
  2. Core Insight
  3. Algorithm Walkthrough
  4. Edge Cases
  5. Full Annotated Code
  6. Interactive Study Demo
  7. Complexity Analysis
Step 01

Brute Force Baseline

Problem summary: Table: patients +-------------+---------+ | Column Name | Type | +-------------+---------+ | patient_id | int | | patient_name| varchar | | age | int | +-------------+---------+ patient_id is the unique identifier for this table. Each row contains information about a patient. Table: covid_tests +-------------+---------+ | Column Name | Type | +-------------+---------+ | test_id | int | | patient_id | int | | test_date | date | | result | varchar | +-------------+---------+ test_id is the unique identifier for this table. Each row represents a COVID test result. The result can be Positive, Negative, or Inconclusive. Write a solution to find patients who have recovered from COVID - patients who tested positive but later tested negative. A patient is considered recovered if they have at least one Positive test followed by at least one Negative test on a later date Calculate the recovery

Baseline thinking

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

Pattern signal: General problem-solving

Example 1

{"headers":{"patients":["patient_id","patient_name","age"],"covid_tests":["test_id","patient_id","test_date","result"]},"rows":{"patients":[[1,"Alice Smith",28],[2,"Bob Johnson",35],[3,"Carol Davis",42],[4,"David Wilson",31],[5,"Emma Brown",29]],"covid_tests":[[1,1,"2023-01-15","Positive"],[2,1,"2023-01-25","Negative"],[3,2,"2023-02-01","Positive"],[4,2,"2023-02-05","Inconclusive"],[5,2,"2023-02-12","Negative"],[6,3,"2023-01-20","Negative"],[7,3,"2023-02-10","Positive"],[8,3,"2023-02-20","Negative"],[9,4,"2023-01-10","Positive"],[10,4,"2023-01-18","Positive"],[11,5,"2023-02-15","Negative"],[12,5,"2023-02-20","Negative"]]}}
Step 02

Core Insight

What unlocks the optimal approach

  • No official hints in dataset. Start from constraints and look for a monotonic or reusable state.
Interview move: turn each hint into an invariant you can check after every iteration/recursion step.
Step 03

Algorithm Walkthrough

Iteration Checklist

  1. Define state (indices, window, stack, map, DP cell, or recursion frame).
  2. Apply one transition step and update the invariant.
  3. Record answer candidate when condition is met.
  4. 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 #3586: Find COVID Recovery Patients
// Auto-generated Java example from py.
class Solution {
    public void exampleSolution() {
    }
}
// Reference (py):
// # Accepted solution for LeetCode #3586: Find COVID Recovery Patients
// import pandas as pd
// 
// 
// def find_covid_recovery_patients(
//     patients: pd.DataFrame, covid_tests: pd.DataFrame
// ) -> pd.DataFrame:
//     covid_tests["test_date"] = pd.to_datetime(covid_tests["test_date"])
// 
//     pos = (
//         covid_tests[covid_tests["result"] == "Positive"]
//         .groupby("patient_id", as_index=False)["test_date"]
//         .min()
//     )
//     pos.rename(columns={"test_date": "first_positive_date"}, inplace=True)
// 
//     neg = covid_tests.merge(pos, on="patient_id")
//     neg = neg[
//         (neg["result"] == "Negative") & (neg["test_date"] > neg["first_positive_date"])
//     ]
//     neg = neg.groupby("patient_id", as_index=False)["test_date"].min()
//     neg.rename(columns={"test_date": "first_negative_date"}, inplace=True)
// 
//     df = pos.merge(neg, on="patient_id")
//     df["recovery_time"] = (
//         df["first_negative_date"] - df["first_positive_date"]
//     ).dt.days
// 
//     out = df.merge(patients, on="patient_id")[
//         ["patient_id", "patient_name", "age", "recovery_time"]
//     ]
//     return out.sort_values(by=["recovery_time", "patient_name"]).reset_index(drop=True)
Step 06

Interactive Study Demo

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

Press Step or Run All to begin.
Step 07

Complexity Analysis

Time
O(n)
Space
O(1)

Approach Breakdown

BRUTE FORCE
O(n²) time
O(1) space

Two nested loops check every pair or subarray. The outer loop fixes a starting point, the inner loop extends or searches. For n elements this gives up to n²/2 operations. No extra space, but the quadratic time is prohibitive for large inputs.

OPTIMIZED
O(n) time
O(1) space

Most array problems have an O(n²) brute force (nested loops) and an O(n) optimal (single pass with clever state tracking). The key is identifying what information to maintain as you scan: a running max, a prefix sum, a hash map of seen values, or two pointers.

Shortcut: If you are using nested loops on an array, there is almost always an O(n) solution. Look for the right auxiliary state.
Coach Notes

Common Mistakes

Review these before coding to avoid predictable interview regressions.

Off-by-one on range boundaries

Wrong move: Loop endpoints miss first/last candidate.

Usually fails on: Fails on minimal arrays and exact-boundary answers.

Fix: Re-derive loops from inclusive/exclusive ranges before coding.