Graph

Question 1

breath-first-search

Answer 1

elegant bfs using list comprehension, also can be used in tree traverse (row by row)
bfs = [target]
visited = set(bfs)
for i in range(K):
bfs = [y for x in bfs for y in links[x] if y not in visited]
visited |= set(bfs)

Question 2

binary matrix, find number of islands

Answer 2

bfs/dfs, keep visited

Question 3

binary matrix, find 0s that are surrounded by all 1s

Answer 3

bfs start from boundaries, if 0 are connected to boundaries, set it to 2

Question 4

dfs

Answer 4

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