Two Pointers

Opposite, same-direction, and fast-slow pointer techniques.

6 min readMastering Data Structures & Algorithms for Software Engineering InterviewsCode

Opposite Pointers

Left pointer at start, right at end. Move toward each other based on comparison. Requires sorted array or known ordering property.

Gotwo-sum-ii---sorted-array.go

16 linesLn 1, Col 1Go

Sorted array → if sum too small, move left right (increase). If too large, move right left (decrease). O(n) vs O(n²) brute force.

Same Direction

Both pointers move left-to-right. Fast pointer scans ahead, slow pointer marks position for writes. Used for in-place modifications, removing duplicates, partitioning.

Goremove-duplicates-from-sorted-array.go

16 linesLn 1, Col 1Go

Slow points to last unique element. Fast scans for new unique values. When found, advance slow and copy.

Fast & Slow Pointers

Slow moves 1 step, fast moves 2 steps. Used for cycle detection (Floyd's algorithm), finding middle of linked list, detecting cycle start.

Gocycle-detection-and-middle-finding.go

47 linesLn 1, Col 1Go

Cycle: fast laps slow if cycle exists. Middle: when fast reaches end, slow at middle. Cycle start: after meeting, reset slow to head, move both 1 step until meet.

When to Use Each

Type	Use When	Requires
Opposite	Pair sum, palindrome, container with most water	Sorted or monotonic property
Same direction	Remove duplicates, merge, partition, Dutch flag	In-place modification OK
Fast/slow	Cycle detection, middle element, cycle start	Linked list or cyclic structure

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