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Word Search [Medium] — DFS Backtracking on Grid
Search for a word in a 2D grid using DFS backtracking with in-place visited marking.
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Grid
30 articles
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BFS & DFS on Graphs and Grids — Complete Guide
Master BFS and DFS on graphs and grids: islands, flood fill, shortest paths, connected components, and multi-source BFS with 5-language implementations.
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Number of Islands — DFS/BFS Flood Fill
Count connected land components in a binary grid using DFS flood fill or BFS. Classic easy problem, foundation for all island variants.
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Flood Fill — DFS Color Replace
Implement the flood fill algorithm (like paint bucket tool): replace all cells of the same color reachable from a starting cell.
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Island Perimeter — Grid Edge Counting
Calculate the perimeter of an island in a binary grid. Each land cell contributes 4 edges minus 2 for each land neighbour.
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Max Area of Island — DFS with Area Count
Find the maximum area of any island in a binary grid. DFS returns the area of each island, track the maximum.
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Surrounded Regions — Boundary DFS
Capture all 'O' regions not connected to the border. Classic boundary DFS: mark safe cells from edges, then flip remaining O to X.
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Rotting Oranges — Multi-Source BFS
Find minimum minutes until all oranges rot. Multi-source BFS: push all initially-rotten oranges into queue simultaneously, BFS layer by layer.
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01 Matrix — Distance to Nearest 0 (Multi-Source BFS)
For each cell find distance to nearest 0. Multi-source BFS from all 0s simultaneously gives optimal O(m*n) solution.
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Walls and Gates — Multi-Source BFS from Gates
Fill each empty room with distance to nearest gate. Multi-source BFS from all gates (0) simultaneously, walls (-1) are barriers.
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Number of Enclaves — Boundary DFS Count
Count land cells that cannot reach the border. Boundary DFS marks all reachable land from borders, then count remaining land cells.
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Count Sub Islands — DFS Subset Check
Count islands in grid2 that are subsets of islands in grid1. DFS the island in grid2 and verify every cell is also land in grid1.
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Making a Large Island — DFS Color Map + Merge
Find the largest island after flipping exactly one 0 to 1. Color each island with DFS, store sizes, then check each 0 cell's unique neighbour islands.
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Pacific Atlantic Water Flow — Two-Source DFS
Find cells that can flow to both Pacific and Atlantic oceans. Run DFS backwards from each ocean border, find intersection.
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Shortest Path in Binary Matrix — BFS 8-Directional
Find shortest clear path from top-left to bottom-right in binary matrix using 8-directional BFS. Return path length or -1.
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As Far from Land as Possible — Multi-Source BFS
Find the water cell farthest from any land. Multi-source BFS from all land cells gives each water cell its min distance to land.
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Word Search — DFS with Backtracking
Search for a word in a grid by moving to adjacent cells without reuse. DFS with backtracking: mark visited, recurse, unmark.
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Number of Distinct Islands — DFS Shape Hashing
Count islands with distinct shapes. Encode each island's DFS traversal path as a string to capture shape, store in a set.
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Number of Closed Islands — Boundary DFS + Count
Count islands fully surrounded by water (no border touch). Flood-fill border land first, then count remaining connected land components.
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Shortest Bridge — DFS Find + BFS Expand
Find minimum flips to connect two islands. DFS to find and color first island, then BFS outward until reaching second island.
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Minimum Effort Path — Dijkstra / Binary Search + BFS
Find path from top-left to bottom-right minimizing maximum absolute difference between consecutive cells. Use Dijkstra or binary search + BFS.
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Number of Islands II — Union-Find Online Queries
Count islands after each addLand operation. Online version requires Union-Find: add each land cell and union with adjacent land cells.
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Nearest Exit from Entrance in Maze — BFS
BFS from entrance in a maze to find nearest exit (border empty cell). Classic BFS shortest path with exit condition.
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Snakes and Ladders — BFS on Board State
Minimum dice rolls to reach square n^2. Convert board position to 2D coordinates (Boustrophedon order), BFS on board states.
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Number of Islands — BFS or DFS Flood Fill
Count islands in a 2D grid using BFS or DFS to flood-fill connected land cells, marking them visited.
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Pacific Atlantic Water Flow — Reverse BFS from Both Oceans
Find cells that can flow to both oceans by doing reverse BFS from each ocean's borders and finding the intersection.
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Word Search II — Trie + Grid DFS Pruning
Find all words from a list in a grid. Build trie from words, DFS on grid while traversing trie simultaneously to prune early.
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Word Search I & II: DFS Backtracking on Grid
Search for words in a character grid using DFS backtracking. Word Search II uses a Trie for simultaneous multi-word search.
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A* Search — Heuristic Shortest Path for Grid Problems
A* combines Dijkstra's correctness with a heuristic to guide search toward the goal. Uses f(n)=g(n)+h(n) priority. Optimal when heuristic is admissible.
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Swim in Rising Water — Dijkstra / Binary Search + BFS
Find the minimum time to swim from (0,0) to (n-1,n-1) where you can only move to a cell when time >= cell value. Two approaches: Dijkstra O(n² log n) and binary search + BFS O(n² log n).
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