217.Contains Duplicate

📘 LeetCode 217: Contains Duplicate

📌 LeetCode: 217.Contains-Duplicate

📋 Problem Statement

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title: “LeetCode 217: Contains Duplicate — Engineering & Algorithmic Solution Note” problem_number: 217 problem_name: “Contains Duplicate” difficulty: “Easy” category: “dsa_solutions” tags: [leetcode-217, contains-duplicate, hash-set, array, csharp, dotnet9, dsa, problem-solving, leetcode, learning-note]

📘 LeetCode 217: Contains Duplicate — Engineering & Algorithmic Solution Note

📌 Connected Resource Graph

• 🟢 Primary Theory Note: 00_overview_hash_tables_dictionary_hashmap.md
• 🟡 Topic Overview Node: 00_overview_array_contiguous_memory.md
• 🎓 Vietnamese DSA Roadmap: 01_DSA/01_theory_and_patterns/Lộ trình học DSA - Nền tảng thuật toán.md
• 📂 Local Project Companion Files:
  • 💻 Program.cs
  • ⚡ complexity.md
  • 🚨 tips.md
  • ✍️ practice.md

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1. 🎯 Problem Statement & Constraints

Given an integer array nums, return true if any value appears at least twice in the array, and return false if every element is distinct.

Examples

Example 1:

Input: nums = [1, 2, 3, 1]
Output: true
Explanation: The element 1 occurs at the indices 0 and 3.

Example 2:

Input: nums = [1, 2, 3, 4]
Output: false
Explanation: All elements are distinct.

Example 3:

Input: nums = [1, 1, 1, 3, 3, 4, 3, 2, 4, 2]
Output: true

Constraints

• $1\le \text{nums.length}\le 10^5$
• $-10^9\le \text{nums}[i]\le 10^9$

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2. 💡 Optimal Implementation (C# Hash Set)

public static bool ContainsDuplicate(int[] nums)
{
    var seen = new HashSet<int>(nums.Length);
    foreach (var num in nums)
    {
        if (!seen.Add(num))
        {
            return true; // Already exists!
        }
    }
    return false;
}

Key Engineering Observations:

1. HashSet<T>.Add() returns false if the item is already present, allowing single-pass early termination.
2. Initializing with new HashSet<int>(nums.Length) prevents costly internal bucket re-allocations for large arrays.

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💡 Multi-Solution Analysis

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title: “Complexity Analysis — LeetCode 217: Contains Duplicate” tags: [leetcode-217, contains-duplicate, complexity, big-o, hash-set, dsa, problem-solving, leetcode, learning-note]

⚡ Complexity Analysis — LeetCode 217: Contains Duplicate

1. Asymptotic Comparison Matrix

Approach	Best Time	Average Time	Worst Time	Space Complexity	Notes
Brute Force (Nested Loops)	$\mathcal{O}(1)$	$\mathcal{O}(N^2)$	$\mathcal{O}(N^2)$	$\mathcal{O}(1)$	TLE on $N=10^5$
Sorting In-Place	$\mathcal{O}(N\log N)$	$\mathcal{O}(N\log N)$	$\mathcal{O}(N\log N)$	$\mathcal{O}(1)$ or $\mathcal{O}(\log N)$	Mutates input array
Hash Set (Optimal)	$\mathcal{O}(1)$	$\mathcal{O}(N)$	$\mathcal{O}(N)$	$\mathcal{O}(N)$	Industry standard, fast lookup

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2. Deep Dive: Hash Set Tradeoffs

• Time Complexity: $\mathcal{O}(N)$ average time. In the worst case where duplicate is at the end or no duplicates exist, we iterate $N$ times with $\mathcal{O}(1)$ set additions.
• Space Complexity: $\mathcal{O}(N)$ additional memory to hold up to $N$ distinct integers in the heap.

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🛠️ C# Tips & Common Pitfalls

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title: “Interview Tips & Traps — LeetCode 217: Contains Duplicate” tags: [leetcode-217, contains-duplicate, interview-tips, edge-cases, dsa, problem-solving, leetcode, learning-note]

🚨 Interview Tips & Traps — LeetCode 217: Contains Duplicate

💡 Edge Cases to Mention to Interviewer

1. Array Length 1: Return false immediately since an array with a single element cannot contain duplicates.
2. All Elements Identical: E.g. [7, 7, 7, 7]. Loop terminates immediately on index 1.
3. Negative Numbers & Zero: Sets properly handle sign bits and 0 without special cases.
4. Memory-Constrained Environments:

   If the interviewer says: “You cannot allocate additional $\mathcal{O}(N)$ heap memory”, pivot to In-Place Sorting ($\mathcal{O}(N\log N)$ time, $\mathcal{O}(1)$ space) and check nums[i] == nums[i-1].

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🏋️ Space to Practice

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title: “practice” tags: [dsa, problem-solving, leetcode, learning-note]

Practice Runs