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Problem solution · Python

Count Cells in Overlapping Horizontal and Vertical Substrings

Count Cells in Overlapping Horizontal and Vertical Substrings: a Python solution using direct simulation. Learn the idea, check the complexity, and read the full code, with credit to walkccc LeetCode Solutions.

Technique
Direct simulation
Source
walkccc LeetCode Solutions
Length
52 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

Direct simulation

For Count Cells in Overlapping Horizontal and Vertical Substrings, the implementation follows the problem’s operations directly while maintaining only the state needed for the next decision.

  1. Translate each rule into one explicit state update.
  2. Maintain the invariant after every processed item.
  3. Return the accumulated state once all relevant input has been handled.

Code notes

  • 52 lines of Python from the credited upstream file 3529.py.
  • The implementation visibly relies on sequence storage.
  • No explicit loop blocks detected.

Complexity

Count the number and nesting of passes over the input, then include the maintained containers in the memory estimate.

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 codeCount Cells in Overlapping Horizontal and Vertical Substrings · PythonPython
Use this to learn the idea, then write your own version.
class Solution:  def countCells(self, grid: list[list[str]], pattern: str) -> int:    BASE = 13    HASH = 1_000_000_007    m = len(grid)    n = len(grid[0])     def markMatchedCells(flattenedGrid: str, isHorizontal: bool) -> list[list[bool]]:      matchMatrix = [[False] * n for _ in range(m)]      matchPrefix = [0] * (len(flattenedGrid) + 1)      pows = [1]  # pows[i] := BASE^i % HASH      patternHash = 0      runningHash = 0       for i in range(1, len(pattern)):        pows.append((pows[-1] * BASE) % HASH)       for c in pattern:        patternHash = (patternHash * BASE + (ord(c) - ord('a'))) % HASH       for i in range(len(flattenedGrid)):        runningHash = (            runningHash * BASE + (ord(flattenedGrid[i]) - ord('a'))) % HASH        if i >= len(pattern) - 1:          if runningHash == patternHash:  # Match found.            matchPrefix[i - len(pattern) + 1] += 1            matchPrefix[i + 1] -= 1          # Remove the contribution of the oldest letter.          oldestLetterHash = (              pows[len(pattern) - 1] *              (ord(flattenedGrid[i - len(pattern) + 1]) - ord('a'))) % HASH          runningHash = (runningHash - oldestLetterHash + HASH) % HASH       for k in range(len(flattenedGrid)):        if k > 0:          matchPrefix[k] += matchPrefix[k - 1]        if matchPrefix[k] > 0:          i = k // n if isHorizontal else k % m          j = k % n if isHorizontal else k // m          matchMatrix[i][j] = True       return matchMatrix     # Find matching positions.    flattenedGridRow = ''.join(cell for row in grid for cell in row)    flattenedGridCol = ''.join(cell for col in zip(*grid) for cell in col)    horizontalMatches = markMatchedCells(flattenedGridRow, True)    verticalMatches = markMatchedCells(flattenedGridCol, False)    return sum(horizontalMatches[i][j] and verticalMatches[i][j]               for i in range(m)               for j in range(n))

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