Two Pointers Technique

Question 1

When is the Two Pointers technique most applicable?

Accepted Answer

Used most for sorted-array pair searches — Two pointers are extremely efficient for searching pairs in a sorted array or checking symmetry in strings.

Question 2

The classic two-pointer 'opposite ends' technique for finding a pair summing to a target requires the input array to be…

Accepted Answer

Arranged in sorted order first — Opposite-ends two-pointer relies on order: if the current sum is too small you move the left pointer right (increase), too large you move the right pointer left (decrease). That monotonic reasoning only works on a sorted array.

Question 3

What is the time complexity of the two-pointer pair-sum search on a sorted array of n elements?

Accepted Answer

O(n) — Each step moves one of the two pointers inward and they never cross back, so the array is scanned at most once: O(n). The brute-force pair check is O(n²).

Question 4

What auxiliary space does the in-place two-pointer technique typically use?

Accepted Answer

O(1) — Two pointers are just two integer indices, so the technique adds O(1) extra space — its main appeal over hash-map approaches that cost O(n).

Question 5

In the 'fast and slow pointer' variant, the two pointers move…

Accepted Answer

Same direction, different speeds — Same-direction (fast/slow) two pointers advance together but at different rates — used for cycle detection, finding the middle of a list, and removing the nth-from-end node.

Question 6

Floyd's cycle-detection ('tortoise and hare') is a two-pointer algorithm. How do the pointers move?

Accepted Answer

Slow by 1, fast by 2 — The slow pointer advances one node per step and the fast pointer two. If a cycle exists they eventually meet; if the fast pointer reaches null, the list is acyclic.

Question 7

For removing duplicates in-place from a SORTED array, the two-pointer pattern uses…

Accepted Answer

A slow write, fast read pointer — The fast pointer scans every element; whenever it finds a value different from the last kept one, the slow 'write' pointer advances and stores it. Result is built in place in O(n).

Question 8

In the 'Container With Most Water' problem, after computing area at the two ends, which pointer should move inward?

Accepted Answer

The pointer at the shorter line — Width shrinks each step, so the only way to possibly increase area is to raise the limiting (shorter) height. Moving the taller side can never help, so move the shorter side's pointer.

Question 9

How does the standard 3Sum solution use two pointers?

Accepted Answer

Fix one, two-pointer scan the rest — Sort, then for each fixed element run an opposite-ends two-pointer search on the remainder for the needed complement. This reduces O(n³) brute force to O(n²).

Question 10

Checking whether a string is a palindrome with two pointers works by…

Accepted Answer

Comparing from both ends inward — Left starts at index 0, right at the end; compare and move both inward. Any mismatch means not a palindrome. O(n) time, O(1) space — no copy needed.

Two Pointers: Valid Palindrome

Palindrome check

Test your knowledge

When is the Two Pointers technique most applicable?

Write the code

Valid Palindrome

Example Cases

The short version

In more depth

Two Pointers: Core Patterns

When to reach for this

Interview Questions

References & Further Reading

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