core-pattern

Two Pointers

Coordinate two indices around a movement invariant to eliminate candidates efficiently in sorted arrays, strings, partitions, and ranges.

Problems

Problems in Two Pointers

Practice problems that share this primary solution pattern and compare the clues that reveal it.

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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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intermediate

12 min read

3Sum Closest

The target does not identify the winning triplet. It tells each pointer which direction is still worth exploring.

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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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intermediate

9 min read

Container With Most Water

The hard part is not calculating width × shorter_height. It is proving why one whole family of pairs can be discarded without checking them.

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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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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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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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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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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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advanced

12 min read

Trapping Rain Water

The picture suggests moving inward from both ends. The proof requires more: finalize only the side whose limiting boundary is already certified.

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A digital tablet showing a web analytics dashboard with graphs and charts.

intermediate

11 min read

Construct Binary Tree from Preorder and Inorder Traversal

The trap in this problem is stopping at “preorder gives the root.” That identifies one node, but not the boundaries of its children. The real solution is…

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intermediate

11 min read

Construct Binary Tree from Inorder and Postorder Traversal

The root is easy to identify. The difficult part is preserving the remaining traversal state while the tree splits into subtrees.

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beginner

9 min read

Convert Sorted Array to Binary Search Tree

Choosing the first or last value as the root preserves sorted order, but it produces a one-sided chain. The key is to choose a root that splits the…

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beginner

10 min read

Pascal's Triangle

The output looks like mathematics. The interview task is simpler: build each row from the row you already have.

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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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intermediate

9 min read

Triangle

The trap is visible: every row offers two choices, so it is tempting to choose the smaller child or enumerate every path. The reliable Triangle solution…

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