Two Pointers Problems You Must Know

Opposite Ends (Sorted Array)

Two pointers at start and end; move based on comparison with target.

Classic problems

• Two Sum (sorted).
• Three Sum: fix one, two pointers for the rest.
• Container with most water.
• Valid palindrome.

Same Direction (Sliding Window Style)

Both pointers move forward; one may "lag" to represent the start of a valid segment.

Examples

• Remove duplicates in-place.
• Remove element in-place.
• Move zeros to end.

Fast and Slow Pointers

Used in linked lists (and sometimes arrays): one pointer moves twice as fast. Great for cycle detection and finding the middle.

Examples

• Linked list cycle detection (Floyd).
• Find middle of linked list.
• Linked list cycle II (find cycle start).

In-Place and Partitioning

• Partition array (e.g. Dutch national flag): multiple pointers to partition into regions.
• Sort colors.

Summary

Two pointers reduce time from O(n²) to O(n). Identify "pairs," "subarray," or "cycle" to choose the variant. Use Preplume(/) to practice.