39 articles
Master bipartite checking and graph coloring: a single BFS or DFS pass that 2-colors a graph and proves it is bipartite, or finds an odd cycle. The pattern behind LeetCode 785 Is Graph Bipartite and LeetCode 886 Possible Bipartition, asked at Google, Meta, and Amazon.
Solve the bottleneck shortest path problem: find a route from source to destination that minimises the maximum edge weight on the path. Combines binary search on the answer with BFS connectivity checks in O((V+E) log W) — a FAANG interview pattern asked at Google and Amazon, and the foundation behind LeetCode 1102 Path With Maximum Minimum Value.
Master BFS and DFS on graphs and grids with the seven core patterns that show up in 90 percent of FAANG graph interviews. Learn flood fill, multi-source BFS, shortest path on unweighted graphs, and connected components with Python and JavaScript code.
Solve LeetCode 200 Number of Islands with DFS flood fill in O(m*n) time. The most asked grid traversal problem at Amazon, Google, and Meta — covers DFS, BFS, and Union-Find approaches with Python and JavaScript.
Solve LeetCode 733 Flood Fill with simple DFS in O(m*n) time. The paint bucket tool from MS Paint reduced to a five-line recursion — the cleanest introduction to grid traversal you can give an interviewer.
Solve LeetCode 463 Island Perimeter in O(m*n) without DFS or BFS. The trick: every land cell contributes 4 edges, minus 2 for each shared edge with another land cell. Pure counting beats traversal.
LC 286 Walls and Gates asks you to fill each empty room with its distance to the nearest gate. The key insight is to flip the direction: instead of BFS from every room, do multi-source BFS from all gates simultaneously and push distances outward in O(m*n) time.
LC 1020 Number of Enclaves asks you to count land cells that cannot reach the grid boundary. The trick is to flip the problem: flood-fill from the boundary inward, eliminating all reachable land, then count what remains.
Find the shortest clear path from top-left to bottom-right in a binary grid with 8-directional movement. Pure BFS on an unweighted graph — the classic shortest-path-in-a-grid interview problem.
Count islands with unique shapes by encoding each DFS traversal path as a string and storing shapes in a set. A classic interview problem testing DFS + hashing.
Count land islands fully surrounded by water (no border touch). Flood-fill border land first to eliminate open islands, then count remaining closed components.
Find the minimum number of 0s to flip to connect two islands. Use DFS to color the first island, then multi-source BFS to expand outward until hitting the second island.
Find the path from top-left to bottom-right that minimizes the maximum absolute difference between consecutive cells. Dijkstra treats effort as edge weight; binary search + BFS checks feasibility for each candidate effort.
BFS from the entrance to find the nearest border empty cell that is not the entrance. Classic BFS shortest-path on a grid with a carefully defined exit condition.
Minimum dice rolls to reach square n² from square 1. The hard part is converting square numbers to board coordinates in Boustrophedon (snake) order. BFS on the state space of squares gives the optimal answer.
Find the minimum number of turns to go from "0000" to the target combination on a 4-wheel lock, avoiding deadend states. Classic BFS on a finite state space — the lock combination is the node, each wheel turn is an edge.
LC 743 Network Delay Time asks for the time a signal takes to reach all n nodes from source k. Single-source shortest path via Dijkstra gives every distance in O(E log V); the answer is the maximum among them.
LC 787 Cheapest Flights Within K Stops asks for the cheapest route from src to dst using at most k intermediate stops. Bellman-Ford with exactly k+1 relaxation rounds handles the stop constraint cleanly without Dijkstra getting confused by the layered state.
LC 127 Word Ladder asks for the shortest sequence of one-letter transformations from beginWord to endWord, where every intermediate word must be in the dictionary. BFS on the implicit word-state graph finds the minimum in O(M^2 * N) using wildcard pattern grouping.
Complete cheatsheet for BFS and DFS on graphs and grids: 7 patterns, complexity table, common pitfalls, and problem index.
Find the kth largest level sum in a binary tree by computing all level sums with BFS and selecting the kth largest using a min-heap — a clean combination of BFS and heap selection.
Week 3 of the FAANG mock program pairs Burst Balloons and Word Ladder II as hard problems under timed pressure. Learn the stuck-recovery protocol, the inversion insight, and why partial credit plus continuous communication beats silence every time.
Solve LeetCode 994 Rotting Oranges step by step using multi-source BFS with a queue. A FAANG interview favorite at Amazon, Google, and Microsoft that teaches level-order traversal, simultaneous infection spread, and grid traversal patterns.
Solve LeetCode 200 Number of Islands using BFS with a queue or DFS with a stack. The most asked grid interview question at Amazon, Google, Meta, and Microsoft, teaching connected components and flood fill.
Solve LeetCode 207 Course Schedule using Kahn algorithm BFS topological sort with a queue and indegree array. A FAANG interview classic at Amazon, Google, and Meta that tests cycle detection in a directed graph.
Solve LeetCode 210 Course Schedule II by returning an actual topological order using Kahn algorithm BFS or DFS with three colors. A FAANG interview classic at Amazon, Google, Meta, and Microsoft.
Solve LeetCode 127 Word Ladder using BFS with a queue and a wildcard pattern map for O(1) neighbor lookups. A FAANG hard at Amazon, Google, Meta, and Microsoft that tests BFS on implicit graphs.
Solve LeetCode 417 Pacific Atlantic Water Flow with reverse BFS from both ocean borders using a deque queue. A FAANG grid traversal classic asked at Amazon, Google, and Meta.
Solve LeetCode 286 Walls and Gates with multi-source BFS from every gate using a deque queue in O(m times n). A FAANG grid traversal classic.
Solve LeetCode 297 Serialize and Deserialize Binary Tree with a BFS queue producing a level-order encoding parsed in O(n). A FAANG hard tree design favorite.
LeetCode 102 — Binary Tree Level Order Traversal, asked at Amazon, Meta, Google and Microsoft. The canonical BFS template that powers Right Side View, Zigzag, Largest in Each Row and 30+ other problems.
LC 116 Populate Next Right Pointers asks you to connect each node to its next right sibling. The O(1) space solution leverages already-connected next pointers on the current level to wire up the next level — a pattern tested at Amazon and Microsoft.
LC 863 All Nodes Distance K in Binary Tree finds all nodes exactly K edges from a target. The key insight is converting the tree to an undirected graph by recording parent pointers, then running BFS from the target — a pattern tested at Amazon and Google.
Solve LeetCode 662 Maximum Width of Binary Tree with the heap-style index trick used in Amazon, Meta, and Microsoft interviews. Includes overflow-safe BFS code in Python and JavaScript.
Solve LeetCode 987 Vertical Order Traversal using DFS coordinates plus tie-breaking sort. A Hard FAANG question asked at Meta, Amazon, and Google.
Verify a binary tree is complete with one BFS pass. Once a null child is encountered, every subsequent dequeued node must be null. LeetCode 958 is a Medium FAANG question asked at Amazon, Meta, and Microsoft.
LC 297 Serialize and Deserialize Binary Tree is a top FAANG hard problem asked at Facebook, Amazon, and Google. The DFS preorder approach with null markers gives a compact O(n) codec; BFS level-order is more intuitive. Both are valid interview answers.
LC 993 Cousins in Binary Tree checks if two nodes are at the same depth with different parents. The clean O(n) BFS solution processes one level at a time — a foundational tree problem tested at Amazon and Microsoft.
LeetCode 720 — find the longest word in a dictionary that can be built one character at a time, with each prefix also in the dictionary. Trie + BFS gives lex-smallest tie-breaking for free.