Core Insight A visited structure makes each vertex’s work happen at most once.
Common Pitfall Mark a node visited when it is scheduled, not after duplicate work is queued.
Description
Find cells that can flow to both Pacific and Atlantic oceans.
Constraints
- Time Complexity:
O(m·n) - Space Complexity:
O(m·n)
Tags
arraydfsbfsmatrix
Implementation Plan Translate the invariant into these small, testable moves.
- Choose a start node or component.
- Add it to the traversal frontier.
- Visit valid unseen neighbors.
- Repeat for every remaining component if needed.
def pacificAtlantic(heights):
ROWS, COLS = len(heights), len(heights[0])
pac, atl = set(), set()
def dfs(r, c, visit, prevHeight):
if ((r, c) in visit or r < 0 or c < 0 or
r == ROWS or c == COLS or heights[r][c] < prevHeight):
return
visit.add((r, c))
dfs(r + 1, c, visit, heights[r][c])
dfs(r - 1, c, visit, heights[r][c])
dfs(r, c + 1, visit, heights[r][c])
dfs(r, c - 1, visit, heights[r][c])
for c in range(COLS):
dfs(0, c, pac, heights[0][c])
dfs(ROWS - 1, c, atl, heights[ROWS - 1][c])
for r in range(ROWS):
dfs(r, 0, pac, heights[r][0])
dfs(r, COLS - 1, atl, heights[r][COLS - 1])
res = []
for r in range(ROWS):
for c in range(COLS):
if (r, c) in pac and (r, c) in atl:
res.append([r, c])
return res