LeetCode Solutions – Ori Levi

This repository contains Java and Python solutions to selected LeetCode problems, each with a short explanation, approach, and complexity analysis.

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📋 Problem Table

#	Title	Level	Java Solution File	Python Solution File	Approaches
1	Two Sum	🟢 Easy	0001_TwoSum.java		HashMap (2 versions)
3	Longest Substring Without Repeating Chars	🟡 Medium	0003_LongestSubstringWithoutRepeating.java		Sliding Window, HashMap, HashSet
15	3Sum	🟡 Medium	0015_3Sum.java	0015_3Sum.py	Sorting + Two Pointers
20	Valid Parentheses	🟢 Easy	0020_ValidParentheses.java		Stack, HashMap
21	Merge Two Sorted Lists	🟢 Easy	0021_MergeTwoSortedLists.java	0021_MergeTwoSortedLists.py	Linked List, Two Pointers, Dummy Head
23	Merge k Sorted Lists	🔴 Hard	0023_MergeKSortedLists.java	0023_MergeKSortedLists.py	Min Heap (PriorityQueue)
33	Search in Rotated Sorted Array	🟡 Medium	0033_SearchInRotatedSortedArray.java	0033_SearchInRotatedSortedArray.py	Binary Search (check sorted half)
42	Trapping Rain Water	🔴 Hard	0042_TrappingRainWater.java		Two Pointers
49	Group Anagrams	🟡 Medium	0049_GroupAnagrams.java		HashMap, Char Sorting
91	Decode Ways	🟡 Medium	0091_DecodeWays.java	0091_DecodeWays.py	Dynamic Programming (DP Array + Rolling)
121	Best Time to Buy and Sell Stock	🟢 Easy	0121_BestTimeToBuyAndSellStock.java	0121_BestTimeToBuyAndSellStock.py	Single Pass, Min Price Tracking
128	Longest Consecutive Sequence	🟡 Medium	0128_LongestConsecutiveSequence.java	0128_LongestConsecutiveSequence.py	HashSet, Start-of-Run Check
200	Number of Islands	🟡 Medium	0200_NumberOfIslands.java		DFS (recursive)
242	Valid Anagram	🟢 Easy	0242_ValidAnagram.java	0242_ValidAnagram.py	Count Array (O(n)), HashMap alternative
347	Top K Frequent Elements	🟡 Medium	0347_TopKFrequentElements.java		HashMap, Min Heap
1004	Max Consecutive Ones III	🟡 Medium	1004_MaxConsecutiveOnesIII.java		Sliding Window

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🧩 Problem 1: Two Sum – Approaches

Approach: One-pass HashMap (recommended) or Pre-insert First Element
Complexity:

• ⏱ Time: O(n)
• 📦 Space: O(n)

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🧩 Problem 3: Longest Substring Without Repeating Characters – Approach

Approach: Sliding Window, HashMap, HashSet
Complexity:

• ⏱ Time: O(n)
• 📦 Space: O(min(n, k))

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🧩 Problem 15: 3Sum – Approach

Approach: Sorting + Two Pointers
Complexity:

• ⏱ Time: O(n²) (sorting O(n log n) + two-pointer per anchor)
• 📦 Space: O(1) extra (ignoring output)

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🧩 Problem 20: Valid Parentheses – Approach

Approach: Stack, HashMap (matching pairs)
Complexity:

• ⏱ Time: O(n)
• 📦 Space: O(n)

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🧩 Problem 21: Merge Two Sorted Lists – Approach

Approach: Linked List, Two Pointers + Dummy Head
Complexity:

• ⏱ Time: O(n + m) — each node is processed once.
• 📦 Space: O(1) — merge is done in-place using existing nodes.

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🧩 Problem 23: Merge k Sorted Lists – Approach

Approach: Min-Heap (PriorityQueue)

• Push all non-null list heads to a min-heap.
• Pop smallest element, append it to result, and push its next (if exists).
  Complexity:
• ⏱ Time: O(N log k), where N = total nodes and k = number of lists
• 📦 Space: O(k) for the heap

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🧩 Problem 33: Search in Rotated Sorted Array – Approach

Approach: Binary Search with “which half is sorted” check

• At each step, one half (left or right) is guaranteed sorted.
• Decide if target lies in the sorted half → shrink search range accordingly.
  Complexity:
• ⏱ Time: O(log n)
• 📦 Space: O(1)

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🧩 Problem 42: Trapping Rain Water – Approach

Approach: Two Pointers with running left/right max
Complexity:

• ⏱ Time: O(n)
• 📦 Space: O(1)

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🧩 Problem 49: Group Anagrams – Approach

Approach: HashMap keyed by sorted-character signature
Complexity:

• ⏱ Time: O(n × k log k)
• 📦 Space: O(n × k)

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🧩 Problem 91: Decode Ways – Approach

Approach: Dynamic Programming
Complexity:

• ⏱ Time: O(n)
• 📦 Space: O(n) or O(1) with rolling variables

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🧩 Problem 121: Best Time to Buy and Sell Stock – Approach

Approach: Single Pass, Min Price Tracking
Complexity:

• ⏱ Time: O(n)
• 📦 Space: O(1)

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🧩 Problem 128: Longest Consecutive Sequence – Approach

Approach: HashSet, Start-of-Run Check
Complexity:

• ⏱ Time: O(n) average (each number visited at most twice)
• 📦 Space: O(n) for the set

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🧩 Problem 200: Number of Islands – Approach

Approach: DFS (recursive) to mark visited land
Complexity:

• ⏱ Time: O(rows × cols)
• 📦 Space: O(rows × cols) recursion stack (worst-case)

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🧩 Problem 242: Valid Anagram – Approach

Approach: Count Array (size 26) or HashMap
Complexity:

• ⏱ Time: O(n)
• 📦 Space: O(1) for fixed alphabet (or O(k) for general Unicode)

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🧩 Problem 347: Top K Frequent Elements – Approach

Approach: HashMap, Min Heap
Complexity:

• ⏱ Time: O(n log k)
• 📦 Space: O(n)

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🧩 Problem 1004: Max Consecutive Ones III – Approach

Approach: Sliding Window
Complexity:

• ⏱ Time: O(n)
• 📦 Space: O(1)