Start hereExpand and shrink a range
Sliding Window
- Choose it when
- Contiguous subarrays and substrings
- Watch for
- Each boundary moves forward while running state avoids rescanning.
Interview patterns
Practice the reusable decisions behind common coding interview problems instead of memorizing isolated solutions.
Which state can be reused instead of recomputed?
Choose by the decision you want to understand
Each lesson names the situation it fits and the behavior worth watching.
BeginnerCoordinate two indices
Two Pointers
- Choose it when
- Pairs, partitions, and in-place edits
- Watch for
- Pointer movement follows a condition that safely removes candidates.
IntermediateMove at different speeds
Fast and Slow Pointers
- Choose it when
- Cycles, middles, and repeated mappings
- Watch for
- Relative speed reveals structure without storing every visited position.
IntermediateKeep a bounded heap
Top K Elements
- Choose it when
- Ranked streams and frequent values
- Watch for
- The heap stores only candidates that can still finish in the top K.
IntermediateCarry cumulative state
Prefix Sum
- Choose it when
- Range sums and subarray counts
- Watch for
- Two cumulative boundaries can replace repeated traversal.
IntermediateSort and sweep ranges
Merge Intervals
- Choose it when
- Scheduling and overlapping spans
- Watch for
- Sorted starts turn overlap detection into one forward pass.