concept

Duplicate Control

Reasoning that prevents, removes, detects, or safely tolerates repeated values or repeated candidate results as a central correctness obligation.

Tagged articles

Duplicate Control articles

21 articles in this tag.

Close-up of hands coding on a laptop, showcasing software development in action.

intermediate

10 min read

3Sum

A reliable 3Sum solution comes from turning a cubic search into a sequence of sorted two-sum scans—and proving why each pointer move is safe.

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Visual abstraction of neural networks in AI technology, featuring data flow and algorithms.

advanced

13 min read

4Sum

Four choices suggest an O(n^4) search. Sorting changes the last two choices into a controlled walk.

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Overhead view of a MacBook laptop on a dark desk, showcasing modern technology and minimalism.

intermediate

12 min read

Combination Sum II

The hard part is not finding combinations that add to the target. It is finding them once while respecting the physical number of occurrences in the input.

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Dark-themed laptop setup with a red glowing keyboard and code on screen, ideal for tech enthusiasts.

intermediate

10 min read

Combination Sum

Treat this as an enumeration problem, not a permutation problem. Sort the candidates, keep combinations in nondecreasing order, recurse from the same index…

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3D rendered abstract brain concept with neural network.

intermediate

11 min read

Combinations

The duplicate-ordering trap is the whole problem: [1, 2] and [2, 1] represent one selection, not two. Build every path in increasing order, and the…

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advanced

11 min read

Distinct Subsequences

Repeated characters create the trap: different selections from s can produce the same visible text in t, and the problem still counts those selections…

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Monochrome image of ancient Roman columns showcasing classical architecture and intricate details.

intermediate

10 min read

Find First and Last Position of Element in Sorted Array

A standard binary search finds a match. This problem asks for the entire matching block. The difference is one boundary decision.

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Vibrant green foliage with ferns and tropical plants bathed in natural sunlight.

advanced

9 min read

First Missing Positive

The array is not just input. Under the right invariant, it becomes its own presence map.

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An IT professional operates a computer in a server room, managing network systems and connected devices.

intermediate

10 min read

Longest Substring Without Repeating Characters

A repeated character is not a reason to restart the scan. It is a reason to move the left boundary to the first position that makes the current window…

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Lush green water plants float in a serene pool at Meise Botanical Garden, Belgium.

expert

14 min read

Median of Two Sorted Arrays

Merging is the obvious solution. It is also disqualified by the runtime requirement. The useful reframe is to search for a cut, not for a value: place…

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A robotic hand holding a spoon above a bowl with keyboard keys, showcasing technology themes.

advanced

13 min read

Minimum Window Substring

The hard part is not moving two pointers. It is preserving the target’s multiplicity while the window changes.

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Black computer cables splayed on a vibrant yellow surface, highlighting technology connection themes.

advanced

11 min read

Next Permutation

The reliable way to solve Next Permutation is to stop thinking in four memorized steps. Read the suffix, identify where it is already maximal, then make…

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A stack of traditional terracotta pots in a Vietnamese pottery workshop, illustrating local craftsmanship.

intermediate

12 min read

Permutations II

When nums = [1, 1, 2], ordinary permutation backtracking treats the two 1 values as different input positions. That creates duplicate value sequences.

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A 3D rendering of a neural network with abstract neuron connections in soft colors.

intermediate

9 min read

Remove Duplicates from Sorted Array II

The trap is treating duplicate removal as a counting problem. The sharper model is an input stream and a compacted result prefix: read every candidate,…

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Close-up of a smartphone resting on an HP laptop, symbolizing modern technology integration.

beginner

9 min read

Remove Duplicates from Sorted Array

The word “remove” is misleading here. You do not need to shrink the Python list or delete values from its tail. You need to compact the distinct values…

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Detailed top view of verdant green leaves showcasing nature's beauty, perfect for botanical themes.

intermediate

11 min read

Remove Duplicates from Sorted List II

The key distinction is easy to miss: this problem removes every node belonging to a repeated value. It does not keep the first occurrence. The solution is…

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A sprawling tree with vibrant green leaves creates a serene outdoor scene.

beginner

9 min read

Remove Duplicates from Sorted List

When a problem says “remove duplicates,” the first instinct is often to reach for a set. That works for an unsorted list, but it misses the key clue here:…

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A laptop displaying an analytics dashboard with real-time data tracking and analysis tools.

intermediate

11 min read

Search in Rotated Sorted Array II

Duplicates turn a clean binary-search decision into an information problem. When nums[left], nums[mid], and nums[right] are equal, you cannot tell which…

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A breathtaking view of a desert landscape with a vibrant sunset illuminating the horizon.

intermediate

12 min read

Subsets II

The duplicate bug comes from treating equal input positions as different decisions. Sort first, then skip equal candidates only when they are siblings at…

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