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Problem solution · Python

Trapping Rain Water II

Trapping Rain Water II: a Python solution using heap or priority queue. Learn the idea, check the complexity, and read the full code, with credit to walkccc LeetCode Solutions.

Technique
Heap or priority queue
Source
walkccc LeetCode Solutions
Length
40 lines
Start with the idea.

Try the problem first. If you get stuck, read the approach below, then write your own solution. The full code is at the bottom.

Approach

Heap or priority queue

For Trapping Rain Water II, the implementation repeatedly takes the currently best candidate from a heap while inserting newly available choices.

  1. Define the priority key and whether the smallest or largest item should lead.
  2. Push each candidate when it becomes eligible.
  3. Discard stale entries when necessary and process the best live candidate.

Code notes

  • 40 lines of Python from the credited upstream file 407.py.
  • The implementation visibly relies on sequence storage, ordered lookup, work queue.
  • No explicit loop blocks detected.

Complexity

Count heap pushes and pops; each normally contributes a logarithmic factor in the heap size.

Check the problem constraints before deciding whether this complexity will pass.

Source

Code and credit

This code comes from walkccc LeetCode Solutions by P.-Y. Chen (walkccc) and is used under the MIT licence.

Open problem ↗Original source ↗
Full codeTrapping Rain Water II · PythonPython
Use this to learn the idea, then write your own version.
class Solution:  def trapRainWater(self, heightMap: list[list[int]]) -> int:    DIRS = ((0, 1), (1, 0), (0, -1), (-1, 0))    m = len(heightMap)    n = len(heightMap[0])    ans = 0    minHeap = []    seen = set()     for i in range(m):      heapq.heappush(minHeap, (heightMap[i][0], i, 0))      heapq.heappush(minHeap, (heightMap[i][n - 1], i, n - 1))      seen.add((i, 0))      seen.add((i, n - 1))     for j in range(1, n - 1):      heapq.heappush(minHeap, (heightMap[0][j], 0, j))      heapq.heappush(minHeap, (heightMap[m - 1][j], m - 1, j))      seen.add((0, j))      seen.add((m - 1, j))     while minHeap:      h, i, j = heapq.heappop(minHeap)      for dx, dy in DIRS:        x = i + dx        y = j + dy        if x < 0 or x == m or y < 0 or y == n:          continue        if (x, y) in seen:          continue        if heightMap[x][y] < h:          ans += h - heightMap[x][y]          # Fill water in grid[x][y].          heapq.heappush(minHeap, (h, x, y))        else:          heapq.heappush(minHeap, (heightMap[x][y], x, y))        seen.add((x, y))     return ans

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