Approach 1Optimal▶ Animated
Second Minimum Time to Reach Destination — graph / tree walk
Each unvisited land cell starts an island; BFS paints the whole component. Built for LeetCode 2045. Second Minimum Time to Reach Destination.
A grid / graph walk is the same idea on trees, mazes and adjacency lists: mark what you touch so you never count it twice, and expand to neighbours.
Algorithm
- Scan every cell.
- On an unvisited
'1', increment the count and BFS-paint the island to'0'.
- Time
- O(mn)
- Space
- O(mn)
class Solution: def secondMinimumTimeToReachDestination(self, grid: list[str]) -> int: if not grid: return 0 rows, cols = len(grid), len(grid[0]) g = [list(row) for row in grid] islands = 0 def flood(sr: int, sc: int) -> None: q = [(sr, sc)] g[sr][sc] = "0" while q: r, c = q.pop(0) for dr, dc in ((1, 0), (-1, 0), (0, 1), (0, -1)): nr, nc = r + dr, c + dc if 0 <= nr < rows and 0 <= nc < cols and g[nr][nc] == "1": g[nr][nc] = "0" q.append((nr, nc)) for r in range(rows): for c in range(cols): if g[r][c] == "1": islands += 1 flood(r, c) return islands