4 queen

N = 4

ld = [0] * 30

rd = [0] * 30

cl = [0] * 30

def printSolution(board):

    for i in range(N):

        for j in range(N):

            print(" Q " if board[i][j] == 1 else " . ", end="")

        print()

def solveNQUtil(board, col):

    if col >= N:

        return True

    for i in range(N):

        if (ld[i - col + N - 1] != 1 and rd[i + col] != 1) and cl[i] != 1:

            board[i][col] = 1

            ld[i - col + N - 1] = rd[i + col] = cl[i] = 1

            if solveNQUtil(board, col + 1):

                return True

            board[i][col] = 0 # BACKTRACK

            ld[i - col + N - 1] = rd[i + col] = cl[i] = 0

    return False

def solveNQ():

    board = [[0 for _ in range(N)] for _ in range(N)]

    if not solveNQUtil(board, 0):

        print("Solution does not exist")

        return False

    printSolution(board)

    return True

if __name__ == "__main__":

    solveNQ()