Pacific Atlantic Water Flow - Leetcode 417 - Python

Pacific Atlantic Water Flow

In this section, the speaker discusses a problem from the Blind 75 list called "Pacific Atlantic Water Flow." They explain how to solve the problem by marking all nodes that can reach the Pacific Ocean or the Atlantic Ocean.

Problem Description

• The problem is called "Pacific Atlantic Water Flow" and is from the Blind 75 list.

• The goal is to mark all cells that can reach either the Pacific Ocean or the Atlantic Ocean.

DFS Algorithm

• Use a Depth First Search (DFS) algorithm to mark cells that can reach either ocean.

• Start at each cell and check if it can flow to both oceans.

• If it can, mark it as reachable.

Implementation Details

• Check if a cell can flow to an ocean by checking its neighboring cells.

• For each cell, check its neighbors in all four directions (up, down, left, right).

• If a neighbor has a higher or equal height than the current cell, then water can flow from the current cell to that neighbor.

• Recursively call DFS on each neighbor that water can flow to.

Boundary Conditions

• Check boundary conditions for each cell before calling DFS on its neighbors.

• If a cell is out of bounds (i.e., row < 0 or column < 0), return immediately.

• If a cell has already been visited, return immediately.

Final Solution

• Mark all cells that can reach either ocean using DFS.

• Return all marked cells as output.

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Understanding Time Complexity

In this section, the speaker discusses the time complexity of a problem and how to approach it recursively.

Recursive Approach

• The starting point for the problem is column 0.

• The time complexity for rows should be n by m squared as big O.

• The position that time complexity starts at is 0.

• Change the starting point to 0 since we are doing this recursively.

DFS Approach

• Doing a lot of repeated work when performing DFS from certain positions.

• Starting DFS from row minus one because it matches over three.

• Changing column to zero in some cases to avoid repeated work.

• This solution works and is about as efficient as possible for this problem in the entire grid.