#3341

Medium

LeetCode Problem Workspace

Find Minimum Time to Reach Last Room I

Find Minimum Time to Reach Last Room I challenges you to determine the minimum time to travel in a dungeon with a grid layout.

array graph heap priority queue matrix shortest path

Problem Statement

You are given a dungeon with n x m rooms arranged as a grid, where each room has a specific opening delay time. You begin at the top-left room (0, 0) at time t = 0, and you must reach the bottom-right room (n - 1, m - 1) as quickly as possible. Moving between adjacent rooms takes exactly one second, but the rooms open after a certain delay represented by the 2D array moveTime.

Each element of moveTime[i][j] represents the minimum time after which the corresponding room (i, j) can be entered. Your task is to find the minimum time required to reach the last room from the starting point using efficient traversal, keeping in mind both the movement time and the room opening times.

Examples

Example 1

Input: moveTime = [[0,4],[4,4]]

Output: 6

The minimum time required is 6 seconds.

Example 2

Input: moveTime = [[0,0,0],[0,0,0]]

Output: 3

The minimum time required is 3 seconds.

Example 3

Input: moveTime = [[0,1],[1,2]]

Output: 3

Example details omitted.

Constraints

2 <= n == moveTime.length <= 50
2 <= m == moveTime[i].length <= 50
0 <= moveTime[i][j] <= 109

Solution Approach

Graph Traversal with Shortest Path Algorithm

This problem is essentially a shortest path problem in a grid, where each cell represents a room. Use a graph traversal technique like Dijkstra's algorithm to calculate the shortest time to each room. Since moving to adjacent rooms takes one second, and the rooms open after a delay, treat the grid as a graph where the edge weights are determined by the room opening times.

Heap-based Priority Queue

To efficiently manage the traversal process, use a min-heap (priority queue) to always expand the room that requires the least time. This ensures that rooms with the lowest time to reach are processed first, which is critical in finding the shortest path in this scenario.

Handling Room Open Times

Each room opens after a specified time, and this must be considered when calculating the overall time. Instead of a simple grid traversal, track the current time for each room and wait for the room to open if needed. This is integrated with the shortest path algorithm to ensure that the total time is minimized.

Complexity Analysis

Metric	Value
Time	O(nm \log(nm))
Space	O(nm)

The time complexity of this solution is O(nm \log(nm)) due to the use of a priority queue (min-heap) for managing room traversal. The space complexity is O(nm) to store the times for each room during the traversal process.

What Interviewers Usually Probe

Look for a clear understanding of graph traversal techniques, particularly shortest path algorithms.
Evaluate if the candidate can implement priority queues efficiently, especially for grid-based problems.
Assess whether the candidate takes room opening times into account while managing the movement time correctly.

Common Pitfalls or Variants

Common pitfalls

Forgetting to account for the room's opening time during traversal, leading to incorrect results.
Not using the priority queue (heap) efficiently, which could result in slower performance for larger grids.
Overcomplicating the solution by not leveraging the shortest path algorithm, leading to inefficient or incorrect solutions.

Follow-up variants

A variant of this problem could involve additional constraints, such as blocked rooms or multiple exits.
You might be asked to solve this problem using a different graph traversal technique, like A* or BFS with optimizations.
Another variant could include more complex room time behaviors, such as dynamic changes in opening times or moving costs.

FAQ

What algorithm should I use to solve the 'Find Minimum Time to Reach Last Room I' problem?

Use a shortest path algorithm like Dijkstra's algorithm with a priority queue to efficiently handle the room opening times and movement constraints.

How can I efficiently manage room opening times in this problem?

You can integrate the room opening times into your traversal logic by updating the current time as you expand to each neighboring room, ensuring that you account for delays.

Why is the priority queue important for this problem?

The priority queue helps ensure that rooms are processed in the order of the least time required to reach them, which is crucial for minimizing the overall time.

What happens if I forget to consider the room opening times?

If you forget to account for the room opening times, the algorithm might incorrectly calculate the time to reach the last room, resulting in an incorrect answer.

Are there any edge cases I should be aware of?

Edge cases could include grids where all rooms are immediately open, grids with the maximum delay times, or grids where the movement time exceeds the opening times of rooms.

terminal

Solution

Solution 1: Dijkstra's Algorithm

We define a two-dimensional array $\textit{dist}$, where $\textit{dist}[i][j]$ represents the minimum time required to reach room $(i, j)$ from the starting point. Initially, we set all elements in the $\textit{dist}$ array to infinity, and then set the $\textit{dist}$ value of the starting point $(0, 0)$ to $0$.

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class Solution:
    def minTimeToReach(self, moveTime: List[List[int]]) -> int:
        n, m = len(moveTime), len(moveTime[0])
        dist = [[inf] * m for _ in range(n)]
        dist[0][0] = 0
        pq = [(0, 0, 0)]
        dirs = (-1, 0, 1, 0, -1)
        while 1:
            d, i, j = heappop(pq)
            if i == n - 1 and j == m - 1:
                return d
            if d > dist[i][j]:
                continue
            for a, b in pairwise(dirs):
                x, y = i + a, j + b
                if 0 <= x < n and 0 <= y < m:
                    t = max(moveTime[x][y], dist[i][j]) + 1
                    if dist[x][y] > t:
                        dist[x][y] = t
                        heappush(pq, (t, x, y))

Continue Practicing

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