Available Captures for Rook Python Solution

LeetCode 999, Easy. Topics: Array, Matrix, Simulation. View on LeetCode. Generate this problem as a practice environment: tested reference solution, 19 parametrized pytest cases, and a playground notebook:

lcpy gen -n 999   # by problem number
lcpy gen -s available_captures_for_rook   # by problem name

Problem

You are given an 8 x 8 matrix representing a chessboard. There is exactly one white rook represented as 'R', some number of white bishops 'B', and some number of black pawns 'p'. Empty squares are represented by '.'. A rook can move any number of squares horizontally or vertically (up, down, left, right) until it reaches another piece or the edge of the board. A rook is attacking a pawn if it can move to the pawn’s square in one move. Note: A rook cannot move through other pieces, such as bishops or pawns. This means a rook cannot attack a pawn if there is another piece blocking the path. Return the number of pawns the white rook is attacking.

Examples

Input: board = [[".",".",".",".",".",".",".","."],[".",".",".","p",".",".",".","."],[".",".",".","R",".",".",".","p"],[".",".",".",".",".",".",".","."],[".",".",".",".",".",".",".","."],[".",".",".","p",".",".",".","."],[".",".",".",".",".",".",".","."],[".",".",".",".",".",".",".","."]]
Output: 3
Explanation: In this example, the rook is attacking all the pawns.

Input: board = [[".",".",".",".",".",".",".","."],[".","p","p","p","p","p",".","."],[".","p","p","B","p","p",".","."],[".","p","B","R","B","p",".","."],[".","p","p","B","p","p",".","."],[".","p","p","p","p","p",".","."],[".",".",".",".",".",".",".","."],[".",".",".",".",".",".",".","."]]
Output: 0
Explanation: The bishops are blocking the rook from attacking any of the pawns.

Input: board = [[".",".",".",".",".",".",".","."],[".",".",".","p",".",".",".","."],[".",".",".","p",".",".",".","."],["p","p",".","R",".","p","B","."],[".",".",".",".",".",".",".","."],[".",".",".","B",".",".",".","."],[".",".",".","p",".",".",".","."],[".",".",".",".",".",".",".","."]]
Output: 3
Explanation: The rook is attacking the pawns at positions b5, d6, and f5.

Constraints

board.length == 8
board[i].length == 8
board[i][j] is either 'R', '.', 'B', or 'p'
There is exactly one cell with board[i][j] == 'R'

Solution

Reference implementation from solution.py on GitHub, full suite in test_solution.py:

class Solution:
    # Time: O(1) — bounded 8x8 scan in four directions
    # Space: O(1)
    def num_rook_captures(self, board: list[list[str]]) -> int:
        rook_i = rook_j = -1
        for i in range(8):
            for j in range(8):
                if board[i][j] == "R":
                    rook_i, rook_j = i, j
        count = 0
        for di, dj in ((1, 0), (-1, 0), (0, 1), (0, -1)):
            i, j = rook_i + di, rook_j + dj
            while 0 <= i < 8 and 0 <= j < 8 and board[i][j] == ".":
                i, j = i + di, j + dj
            if 0 <= i < 8 and 0 <= j < 8 and board[i][j] == "p":
                count += 1
        return count

Complexity

Time	Space
O(1) — bounded 8x8 scan in four directions	O(1)

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Available Captures for Rook Python Solution

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