Algorithms / Grid Graph Traversal
Least You Need to Know: Grid Traversal, Flood Fill, and Boundary Checks
Many interview grid problems are just implicit graph traversal: each cell is a node, legal moves define edges, and BFS or DFS visits connected components or shortest paths. The main implementation risk is careful neighbor and boundary handling.
جو کم از کم جاننا ضروری ہے
- A grid becomes a graph when each cell connects to its legal neighbors.
- Flood fill is component traversal over cells that meet a condition.
- Boundary checks must happen before reading or enqueueing a neighbor cell.
- Marking visited prevents repeated exploration and infinite revisits.
- Counting islands is counting connected components in an implicit grid graph.
اہم علامتیں
(r, c)
row and column state
4-neighbors
up, down, left, right
component
maximal connected region under the movement rule
مختصر حل شدہ مثال
- To count islands, scan the grid for an unvisited land cell.
- Each time you find one, start DFS or BFS and mark its whole connected land region.
- That one traversal claims exactly one island.
- Continue scanning until all land cells are assigned to some traversal.
عام غلطیاں
- Students often forget to check bounds before reading neighbors.
- Students often mutate or revisit cells in inconsistent order and double-count regions.
- Students often choose DFS for shortest path in an unweighted maze when BFS is the better fit.
اس قسم کے سوال کو کیسے پہچانیں
- The input is a matrix with legal moves to neighboring cells.
- You need to count regions, spread through a region, or find a shortest route on equal-cost moves.
- The graph is implicit; neighbors are generated from coordinates rather than listed explicitly.
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