intermediate
12 min read
3Sum Closest
The target does not identify the winning triplet. It tells each pointer which direction is still worth exploring.
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Intermediate
Articles whose authoritative difficulty is intermediate.
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Intermediate articles
62 articles in this tag.
intermediate
10 min read
3Sum
A reliable 3Sum solution comes from turning a cubic search into a sequence of sorted two-sum scans—and proving why each pointer move is safe.
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intermediate
10 min read
Add Two Numbers
The lists already expose digits in the order addition needs. Scan both lists together, track one carry, and keep going until there is no digit or carry…
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intermediate
11 min read
Binary Tree Level Order Traversal II
The output is bottom-up, but the traversal should still be top-down: collect ordinary BFS levels, then reverse the completed groups.
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intermediate
10 min read
Binary Tree Level Order Traversal
If you visit every node but mix adjacent depths, the traversal is still wrong. The key is to make the queue represent one level at a time.
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intermediate
9 min read
Binary Tree Zigzag Level Order Traversal
The queue does not need to zigzag. It only needs to preserve a predictable order while we place each value into its correct output position.
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intermediate
12 min read
Combination Sum II
The hard part is not finding combinations that add to the target. It is finding them once while respecting the physical number of occurrences in the input.
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intermediate
10 min read
Combination Sum
Treat this as an enumeration problem, not a permutation problem. Sort the candidates, keep combinations in nondecreasing order, recurse from the same index…
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intermediate
11 min read
Combinations
The duplicate-ordering trap is the whole problem: [1, 2] and [2, 1] represent one selection, not two. Build every path in increasing order, and the…
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intermediate
11 min read
Construct Binary Tree from Inorder and Postorder Traversal
The root is easy to identify. The difficult part is preserving the remaining traversal state while the tree splits into subtrees.
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intermediate
11 min read
Construct Binary Tree from Preorder and Inorder Traversal
The trap in this problem is stopping at “preorder gives the root.” That identifies one node, but not the boundaries of its children. The real solution is…
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intermediate
9 min read
Container With Most Water
The hard part is not calculating width × shorter_height. It is proving why one whole family of pairs can be discarded without checking them.
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intermediate
12 min read
Convert Sorted List to Binary Search Tree
A sorted array gives you the middle value immediately. A singly linked list makes you walk for it. The better solution avoids the walk entirely: let…
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intermediate
10 min read
Count and Say
The Count and Say solution is a repeated state transition: start with "1", scan the current string into maximal consecutive runs, and emit each run as…
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intermediate
11 min read
Decode Ways
The recurrence resembles Fibonacci, but zeros can remove transitions entirely. Derive the valid-token transitions first; the dynamic program then follows…
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intermediate
10 min read
Find First and Last Position of Element in Sorted Array
A standard binary search finds a match. This problem asks for the entire matching block. The difference is one boundary decision.
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intermediate
10 min read
Flatten Binary Tree to Linked List
It places the left subtree in the correct position, but it can disconnect the original right subtree. The real task is to move the left subtree in front of…
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intermediate
10 min read
Generate Parentheses
Generate only prefixes that can still become valid. The balance state tells you exactly which branches to keep.
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intermediate
11 min read
Gray Code
Gray code looks like a permutation problem, but the useful signal is more specific: enumerate every n-bit state so that each move flips exactly one bit,…
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intermediate
9 min read
Group Anagrams
A useful Group Anagrams solution does not compare every string with every existing group. It assigns each string a stable identity based on its character…
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intermediate
10 min read
Insert Interval
The common trap in the Insert Interval solution is treating the task as generic sorting followed by generic interval merging. That works, but it ignores…
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intermediate
11 min read
Jump Game II
The common mistake in a Jump Game II solution is to commit too early: “From this index, which landing position should I choose?” That creates a path-search…
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intermediate
10 min read
Jump Game
The wrong mental model is a tree of jump paths. The useful model is a moving boundary: the farthest index reachable by any valid path found so far.
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intermediate
11 min read
Longest Palindromic Substring
The difficult part is not recognizing a palindrome. It is preserving contiguity while avoiding repeated work.
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intermediate
10 min read
Longest Substring Without Repeating Characters
A repeated character is not a reason to restart the scan. It is a reason to move the left boundary to the first position that makes the current window…
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intermediate
10 min read
Maximum Subarray
Resetting a running sum to zero looks like the obvious solution—until the array contains only negative numbers. Then the algorithm can quietly return an…
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intermediate
11 min read
Merge Intervals
Pairwise merging feels natural until one merge creates a new overlap with a third interval. Then the bookkeeping branches, earlier comparisons become…
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intermediate
11 min read
Minimum Path Sum
The right Minimum Path Sum solution is a two-dimensional dynamic program. For every coordinate, store the minimum sum needed to reach it from the top-left.…
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intermediate
10 min read
Multiply Strings
Treat the product as a fixed array of decimal positions. Every digit pair has a predictable destination; carry normalization keeps those positions valid.
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intermediate
10 min read
Partition List
A linked-list partition fails in one of two ways: it loses the unread suffix, or it preserves a stale link and creates the wrong structure. The reliable…
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intermediate
10 min read
Path Sum II
That is the central trap in Path Sum II. An internal node may bring the running sum to targetSum, but its path is still incomplete if the node has a child.…
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intermediate
12 min read
Permutations II
When nums = [1, 1, 2], ordinary permutation backtracking treats the two 1 values as different input positions. That creates duplicate value sequences.
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intermediate
11 min read
Permutations
The search tree is easy to recognize and easy to corrupt. Build one position at a time, choose an unused value, recurse, then undo exactly that choice.
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intermediate
10 min read
Populating Next Right Pointers in Each Node II
The queue solution is easy to derive. The constant-space solution comes from reusing the next pointers you are building as the queue for the next level.
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intermediate
12 min read
Populating Next Right Pointers in Each Node
The queue-based solution is easy to see. The constant-space solution is easier to miss: once one level is connected, its next pointers become the queue for…
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intermediate
11 min read
Pow(x, n)
A linear multiplication chain follows the definition of a power. It also ignores the only fact that matters at interview scale: the exponent can be halved.
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intermediate
9 min read
Remove Duplicates from Sorted Array II
The trap is treating duplicate removal as a counting problem. The sharper model is an input stream and a compacted result prefix: read every candidate,…
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intermediate
11 min read
Remove Duplicates from Sorted List II
The key distinction is easy to miss: this problem removes every node belonging to a repeated value. It does not keep the first occurrence. The solution is…
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intermediate
10 min read
Remove Nth Node From End of List
The target is named from the end, but a singly linked list only lets you move forward. The key move is to convert that backward-looking position into a…
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intermediate
10 min read
Restore IP Addresses
The trap is to think “place three dots.” The useful model is narrower: choose exactly four contiguous digit segments, validate each one immediately, and…
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intermediate
11 min read
Reverse Integer
Reversing digits is easy. Reversing them without ever creating an unsafe intermediate value is the interview problem.
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intermediate
12 min read
Reverse Linked List II
A localized reversal fails at the boundaries: the middle looks correct, but the prefix disappears, the suffix becomes unreachable, or the returned head is…
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intermediate
10 min read
Rotate Image
The hard part of rotating a matrix is not visualizing the turn. It is moving every value without destroying one that has not moved yet.
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intermediate
11 min read
Rotate List
A right rotation looks repetitive when described one node at a time. The useful implementation is one split, one reconnection, and one cut.
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intermediate
10 min read
Search a 2D Matrix
A matrix can be two-dimensional storage with a one-dimensional search space. Prove that shape first, then run ordinary binary search over virtual indices.
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intermediate
11 min read
Search in Rotated Sorted Array II
Duplicates turn a clean binary-search decision into an information problem. When nums[left], nums[mid], and nums[right] are equal, you cannot tell which…
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intermediate
12 min read
Search in Rotated Sorted Array
Rotation breaks global order, not all order. Find the sorted half, test its value range, and discard what that range proves impossible.
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intermediate
12 min read
Set Matrix Zeroes
The dangerous part is not writing zeroes. It is remembering which zeroes were causes and which zeroes were created by your own writes.
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intermediate
9 min read
Simplify Path
Treating this as string cleanup is how you lose the problem. Removing every dot breaks valid names such as ..., while careless parent handling can let /../…
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intermediate
11 min read
Sort Colors
The trap in the Sort Colors solution is assuming that “only three values” makes the problem trivial. Counting works. The interview version asks you to see…
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intermediate
8 min read
Spiral Matrix II
The key distinction is simple: Spiral Matrix reads values from an existing grid; Spiral Matrix II constructs the grid while the spiral advances. The…
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intermediate
10 min read
Spiral Matrix
A spiral traversal can look correct on a square matrix and still fail immediately on a single row or column. The reliable model is a shrinking rectangle:…
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intermediate
10 min read
String to Integer (atoi)
A reliable atoi parser is a small state machine: skip leading spaces, read one optional sign, consume one numeric prefix, and guard every accumulator…
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intermediate
12 min read
Subsets II
The duplicate bug comes from treating equal input positions as different decisions. Sort first, then skip equal candidates only when they are siblings at…
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intermediate
11 min read
Swap Nodes in Pairs
A value swap can produce the right sequence while violating the contract. The real task is to move node identities by changing links—and to do it without…
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intermediate
9 min read
Triangle
The trap is visible: every row offers two choices, so it is tempting to choose the smaller child or enumerate every path. The reliable Triangle solution…
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intermediate
12 min read
Unique Paths II
The recurrence is familiar. The interview usually turns on the cells you forgot: a blocked start, a blocked destination, or an obstacle that permanently…
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intermediate
10 min read
Unique Paths
The reliable way to solve Unique Paths is to count paths to each cell, not to enumerate complete routes. Every cell has at most two meaningful…
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intermediate
8 min read
Valid Sudoku
A Sudoku validator does not solve the puzzle. It tracks whether the digits already placed violate any row, column, or 3×3 box constraint.
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intermediate
11 min read
Validate Binary Search Tree
A BST validator must remember the ancestors that still constrain the current node.
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intermediate
14 min read
Word Search
A grid DFS can match the right letters and still be wrong. The missing piece is path-local state: mark a cell when you enter it, explore from that choice,…
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intermediate
11 min read
Zigzag Conversion
A visual zigzag is easy to draw and surprisingly easy to implement incorrectly. The reliable solution is smaller: track the current row, track the movement…
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