technique

In-Place Mutation

Algorithms whose central requirement or advantage is transforming an input structure in place while preserving required data, ordering, reachability, or region invariants.

Tagged articles

In-Place Mutation articles

24 articles in this tag.

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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Beautiful handmade terracotta pots displayed on shelves in an outdoor market, showcasing artisanal craftsmanship.

intermediate

10 min read

Flatten Binary Tree to Linked List

It places the left subtree in the correct position, but it can disconnect the original right subtree. The real task is to move the left subtree in front of…

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Close-up of a digital interface showcasing futuristic graphs and data analytics in low light.

beginner

8 min read

Merge Sorted Array

A left-to-right merge can overwrite values in nums1 before you have compared them. The reliable Merge Sorted Array solution uses backward two pointers:…

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Group of high school students focused on learning in a computer lab setting.

beginner

10 min read

Merge Two Sorted Lists

The common failure mode here is treating linked lists like arrays: copy the values, sort them, and rebuild. That throws away the structure the problem…

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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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Close-up of a moss-covered tree trunk in a dark, moody forest setting.

intermediate

10 min read

Partition List

A linked-list partition fails in one of two ways: it loses the unread suffix, or it preserves a stale link and creates the wrong structure. The reliable…

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Modern minimalist workspace with a laptop, ceramic mug, and glass water bottle on a white surface.

beginner

9 min read

Pascal's Triangle II

The requested output is one row, not the entire triangle. Build that row with one working list, and update it from right to left so each calculation still…

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A peaceful upward view of green tree branches extending towards a vibrant blue sky in spring.

intermediate

10 min read

Populating Next Right Pointers in Each Node II

The queue solution is easy to derive. The constant-space solution comes from reusing the next pointers you are building as the queue for the next level.

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View of the ancient Odeon of Herodes Atticus in Athens with a cityscape backdrop.

intermediate

12 min read

Populating Next Right Pointers in Each Node

The queue-based solution is easy to see. The constant-space solution is easier to miss: once one level is connected, its next pointers become the queue for…

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Detailed view of a sewing machine needle stitching fabric, highlighting sewing process and textile technology.

advanced

12 min read

Recover Binary Search Tree

A binary search tree has a structural invariant that is more useful than its parent-child relationships: inorder traversal visits values in sorted order.…

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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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Professional business meeting with presentation and data analytics on whiteboard.

beginner

8 min read

Remove Element

The array is not shortened. The judge inspects only a prefix, so the job is to compact the values you keep into that prefix and return its length.

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Close-up of blue ethernet cables hanging in a data center, highlighting technology connections.

intermediate

10 min read

Remove Nth Node From End of List

The target is named from the end, but a singly linked list only lets you move forward. The key move is to convert that backward-looking position into a…

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Silhouette of a person walking on a vast sand dune in the desert at sunset in Huacachina, Peru.

intermediate

12 min read

Reverse Linked List II

A localized reversal fails at the boundaries: the middle looks correct, but the prefix disappears, the suffix becomes unreachable, or the returned head is…

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Detailed view of ancient stone columns and arches in Athens, showcasing Greek architecture.

advanced

13 min read

Reverse Nodes in k-Group

Reversing a linked-list segment is easy. Preserving everything on both sides of that segment is the real interview problem.

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From above of green leaves on thin branches of plant growing in botanical garden

intermediate

10 min read

Rotate Image

The hard part of rotating a matrix is not visualizing the turn. It is moving every value without destroying one that has not moved yet.

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Factory chimneys emitting smoke over rooftops on a clear day, illustrating urban pollution.

intermediate

11 min read

Rotate List

A right rotation looks repetitive when described one node at a time. The useful implementation is one split, one reconnection, and one cut.

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Close-up of a tablet displaying analytics charts on a wooden office desk, alongside a smartphone and coffee cup.

intermediate

12 min read

Set Matrix Zeroes

The dangerous part is not writing zeroes. It is remembering which zeroes were causes and which zeroes were created by your own writes.

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Aerial photograph of a tractor working in a vast, vibrant green field captured from above.

intermediate

11 min read

Sort Colors

The trap in the Sort Colors solution is assuming that “only three values” makes the problem trivial. Counting works. The interview version asks you to see…

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System with various wires managing access to centralized resource of server in data center

expert

14 min read

Sudoku Solver

The phrase “try digits and backtrack” is the easy part. The interview-grade solution keeps four representations synchronized: the board, row constraints,…

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A serene beach with soft sand, gentle waves, and lush green trees. Perfect for a nature escape.

intermediate

11 min read

Swap Nodes in Pairs

A value swap can produce the right sequence while violating the contract. The real task is to move node identities by changing links—and to do it without…

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