#two pointers

61 two pointers interview questions, asked at Meta, Microsoft, Google and 15 more companies. Last asked Sep 2026.

Squares of a sorted array, then the k smallest squaresCodingYou get an integer array sorted in non-decreasing order; it may hold negative numbers. 1. Return the squares of all its elements in sorted order, faster than sorting again (O(n)…
Last asked Sep 2026
String compressionCodingGiven a non-empty list of single characters, compress it with run-length encoding: for each run of equal characters write the character, followed by the run's length when it is…
Last asked Sep 2026
Valid word abbreviationCodingA word can be abbreviated by replacing non-overlapping, non-adjacent substrings with their lengths, so internationalization may become i18n or i5a11o1. Given a word and an…
Last asked Sep 2026
Two sumCodingGiven an integer array and a target, return the indices of the two different elements that add up to the target; exactly one such pair exists. Then solve it without a hash map…
Last asked Sep 2026
Does a seven-segment display read the same rotated 180 degrees?CodingAn LED display shows a row of digits on seven-segment cells. Decide whether the display reads exactly the same after rotating it by 180 degrees. Work out which digits map to which…
Last asked Sep 2026
Sort a transformed arrayCodingGiven a sorted array and the coefficients a, b, c of f(x) = ax^2 + bx + c, apply f to every element and return the results in sorted order in O(n). Use two pointers from both…
Last asked Sep 2026
+1
Three sum: unique value triplesCodingReturn every unique triple of values from an integer array that sums to zero (or to a given target), with no duplicate triples in the output. Sort, then fix one value and move two…
Last asked Sep 2026
Valid palindrome ignoring case and punctuationCodingDecide whether a string reads the same forwards and backwards when you ignore letter case and every character that is not a letter or a digit.
Last asked Aug 2026
Rearrange an array so positive and negative numbers alternateCodingGiven an array with positive and negative numbers, reorder it so positives and negatives alternate. Clarify whether relative order must be kept and what to do when one sign has…
Last asked Aug 2026
Zipline between two trees maximizing height times spanCodingTrees stand one unit apart with heights heightsi. A zipline joins two different trees at one height, no higher than the shorter tree. Its value is that height times the distance…
Last asked Aug 2026
+2
Merge two sorted interval listsCodingImplement mergetwointervallists(a, b). Each argument is a list of intervals start, end with integer endpoints and start <= end. Each list is sorted by start, and no two intervals…
Last asked Aug 2026
Longest run of identical consecutive itemsCodingGiven a sequence, such as a string or the list of shows a user watched in order, return the longest run of the same item repeated back to back: its length and the item (and its…
Last asked Jul 2026
Find dictionary words that stretch into a given stringCodingA word can produce a string by repeating some of its letters in place: help can produce heellp. Given the string and a dictionary, return every dictionary word that could produce…
Last asked Jun 2026
Valid palindrome after deleting at most one characterCodingReturn whether a string can become a palindrome by deleting at most one character. Walk two pointers inward and, at the first mismatch, try skipping either side.
Last asked Jun 2026
Sort colors in placeCodingYou get an array of records, each tagged red, green or blue. Sort it in place so all reds come first, then greens, then blues, in one pass and constant extra space (the Dutch…
Last asked Jun 2026
Detect and break a cycle in a linked listCodingDecide whether a singly linked list has a cycle, find the node where the cycle begins, and break the cycle so the list ends.
Last asked Jun 2026
Interval list intersectionsCodingGiven two lists of closed intervals, each sorted and non-overlapping within itself, return every intersection between an interval of the first list and one of the second.
Last asked May 2026
Count palindromic substringsCodingCount the substrings of a string that are palindromes, counting each position separately. Expanding around every centre gives O(n^2) time and O(1) space.
Last asked May 2026
Unique pairs with a target sumCodingReturn each distinct pair of values from an array that sums to a target, counting a pair only once even when the values repeat.
Last asked May 2026
Merge three sorted arrays without duplicatesCodingMerge three sorted integer arrays into one sorted array that contains each value once. Walk the three arrays together in a single pass rather than concatenating and sorting.
Last asked Apr 2026
+1
3SumCodingReturn every unique triple of values in an integer array that adds up to zero, with no duplicate triples in the answer. Aim for O(n^2) time after sorting.
Last asked Apr 2026
Sort colors (three-way partition in place)CodingAn array holds only three kinds of values, such as 0, 1 and 2. Sort it in place in one pass without a counting sort. Explain the Dutch national flag partition and its invariants.
Last asked Apr 2026
Remove duplicates from a sorted array in placeCodingRemove duplicates from a sorted array in place so each value appears once, and return the new length; the first part of the array must hold the unique values in order. Use O(1)…
Last asked Apr 2026
One edit distanceCodingReturn whether two strings are exactly one edit apart, where an edit inserts, deletes or replaces one character. Solve it in one pass and clarify the edge cases (equal strings…
Last asked Apr 2026
Palindrome linked list without modifying the listCodingDecide whether a singly linked list reads the same both ways. Do it without changing the list, or restore it exactly if you reverse part of it. Compare the stack approach with…
Last asked Apr 2026
+1
Balanced prefixes in a permutationCodingYou are given a permutation p of length n: every integer from 1 to n appears exactly once, in some order. Call a number k (with 1 <= k <= n) balanced if some contiguous subarray…
Last asked Apr 2026
+2
Shortest subarray with at least k distinct valuesCodingYou are given an integer array arr and an integer k. Call a contiguous subarray good if it contains at least k different values. Return the length of the shortest good subarray…
Last asked Apr 2026
Can one board be turned into another by moving pieces?CodingA board is a string of R, L and empty cells. R pieces can only move right and L pieces only left, one empty cell at a time, and no piece can jump over another. Decide whether a…
Last asked Apr 2026
Research OA: best value ratio, shuttle trips and grid commandsCodingA multi-part online assessment. 1. Given ratings and prices, return the index with the highest rating-to-price ratio, choosing the smaller index on ties. Compare the fractions…
Last asked Mar 2026
Score how close a string is to a palindromeCodingDesign a score for strings that are almost palindromes and implement it. Decide what the score measures (for example, mismatched pairs when comparing from both ends, or the fewest…
Last asked Mar 2026
Dot product of two sparse vectorsCodingStore very large, mostly-zero vectors compactly and compute the dot product of two of them. Compare a hash map with sorted index-value pairs, and say how you would reuse work when…
Last asked Feb 2026
Merge two sorted arrays in placeCodingTwo integer arrays, arr1 and arr2, are each sorted in ascending order. Merge arr2 into arr1 without using another array, so that arr1 ends up holding all the values, still…
Last asked Feb 2026
Intersection of two linked lists that may have cyclesCodingGiven the heads of two singly linked lists, each of which may or may not contain a cycle, decide whether they share a node and return the first shared node. Find where each list's…
Last asked Jan 2026
+1
Dot product of run-length encoded vectorsCodingA very long integer vector has many repeated values. Design a compact representation for it (for example, runs of value and count), then compute the dot product of two vectors…
Last asked Jan 2026
Objects seen on both days by at least two clientsCodingTwo log files, one per day, hold lines of timestamp, objectid, clientid. Return the object ids that appear on both days and were accessed by at least two different clients. Then:…
Last asked Nov 2025
Valid palindrome IICodingReturn whether a string can become a palindrome after deleting at most one character. Walk two pointers inward; at the first mismatch try skipping either side once.
Last asked Oct 2025
Trapping rain waterCodingGiven bar heights in an elevation map, each of width 1, compute how much water is trapped after rain. Start with the brute-force solution and its complexity, then improve it to…
Last asked Sep 2025
Merge sorted arrayCodingMerge a sorted array into another sorted array that has room at its end, in place, and give the complexity. Follow-up reported: could you merge starting from the front of each…
Last asked Sep 2025
Is a sequence a subsequence of a tree's in-order traversal?CodingGiven a binary tree and a sequence of numbers, decide whether the sequence appears, in order, within the tree's in-order traversal (not necessarily next to each other). Walk the…
Last asked Sep 2025
Earliest common slot in two calendarsCodingGiven two people's calendars as lists of time intervals and a meeting length, return the earliest slot of that length that works for both, or nothing. Clarify whether the…
Last asked Sep 2025
Order jumps between blocks to burn the most caloriesCodingYou stand on the ground (height 0) next to n blocks of given heights. You must jump onto every block exactly once, in any order, and you never return to the ground. A jump from…
Last asked Sep 2025
Shortest unsorted continuous subarrayCodingFind the shortest contiguous subarray that, if sorted, makes the whole array sorted, and return its length. Start from sorting a copy, then find an O(n) two-pointer solution.
Last asked Aug 2025
Nth node from the end of a linked listCodingReturn (or remove) the n-th node from the end of a singly linked list in one pass, moving a lead pointer n steps ahead of a trailing one.
Last asked Aug 2025
Compare two linked lists of string chunksCodingTwo singly linked lists each hold pieces of text, one piece per node (in the reported version, one character per node stored as a string). Decide whether both lists spell the same…
Last asked Jul 2025
Valid palindrome and its variantsCodingDecide whether a string is a palindrome after ignoring case and anything that is not a letter or digit. Then solve variants: allow deleting at most one character, or check without…
Last asked Jul 2025
Sum of the longest run of positive numbersCodingAn array holds positive and negative numbers. Find the longest contiguous run of positive numbers and return its sum (decide and state how ties between equally long runs are…
Last asked Jul 2025
Can left and right pieces slide into a target stringCodingA string holds pieces that can only move left, pieces that can only move right, blanks, and walls that nothing passes. Given a start and a target string, decide whether the start…
Last asked Jul 2025
Shortest distance between X and Y in a stringCodingFind the shortest distance between characters X and Y in a string in one pass. Reported on 1point3acres' Glean bank; reconstructed in our own words.
Minimum swaps to put evens before oddsCodingGiven an integer array nums of length n, you want every even value to come before every odd value. In one move you may swap the values at any two distinct indices. Return the…
Minimum flips to make every row and column a palindromeCodingYou are given an N x M grid as a list of strings. Each cell is either "B" (black) or "W" (white). A row is symmetric if it reads the same left to right as right to left. A column…
Maximum tests in one activation windowCodingA shared test environment receives test cases over time. You are given an integer array arrivalTime, where each entry is the time one test case arrives. Several test cases can…
Levels cleared from every starting pointCodingA game has n levels in a row. You are given two integer arrays of length n, layers and energy, and an integer K. A run that starts at level i begins with exactly K energy and…
Longest substring without three repeats in a rowCodingGiven a string s, return the length of the longest substring of s in which no character appears three or more times in a row. Call such a substring semi-alternating: runs of one…
+2
Largest value shared by two BSTsCodingYou are given the roots of two binary search trees, a and b, holding integer values. Return the largest integer that appears in both trees. If the trees share no value, return -1…
Best starting layer for a falling ball with an energy budgetCodingA ball falls through a tower of n layers, numbered 0 (top) to n - 1 (bottom). You get two integer arrays of length n and one integer: - costsi (= 0): the energy the ball spends to…
Fewest append-to-end moves to reverse a binary imageCodingAn image is stored as a binary string s: '1' is a white pixel and '0' is a black pixel. To make a backup, you want to turn s into its reverse, s read from right to left. You may…
Which prefixes of 1..n appear as a contiguous blockCodingYou are given an array p of length n that is a permutation of the integers 1..n (each value appears exactly once). For every k from 1 to n, decide whether p has a contiguous…
Password similarity under cyclic character bumpsCodingA security team wants to reject a new password when it is too close to the user's previous one. You may transform the new password like this: pick any set of its positions…
Longest run of customers a bank can serveCodingA bank opens with initial dollars in its vault (initial = 0). Customers wait in a fixed queue, given as an integer array transactions: a positive value is a deposit and a negative…
+1
Merge two sorted arrays into distinct valuesCodingTwo integer arrays, nums1 and nums2, are each sorted in non-decreasing order and may contain repeated values. Return one array, sorted in ascending order, that contains every…
Cross the most walls on a number line with limited energyCodingWalls stand at distinct non-zero integer points on an infinite number line. Wall k is at positionsk and costs costsk = 0 energy to cross. You start at 0 with energy units. Each…