N Queens.py

# N Queens Problem - User-defined N

def print_solution(board, N):
    for i in range(N):
        for j in range(N):
            print(board[i][j], end=' ')
        print()
    print()  # blank line between solutions

def is_safe(board, row, col, N):
    # Check this row on left side
    for i in range(col):
        if board[row][i] == 1:
            return False
    # Check upper diagonal on left side
    for i, j in zip(range(row, -1, -1), range(col, -1, -1)):
        if board[i][j] == 1:
            return False
    # Check lower diagonal on left side
    for i, j in zip(range(row, N, 1), range(col, -1, -1)):
        if board[i][j] == 1:
            return False
    return True

def solve_nq_util(board, col, N):
    # Base case: If all queens are placed
    if col >= N:
        print_solution(board, N)
        return True

    res = False
    for i in range(N):
        if is_safe(board, i, col, N):
            board[i][col] = 1
            res = solve_nq_util(board, col + 1, N) or res
            board[i][col] = 0  # Backtrack
    return res

def solve_nq(N):
    board = [[0 for _ in range(N)] for _ in range(N)]
    if not solve_nq_util(board, 0, N):
        print("No solution exists for N =", N)

# --- Main program ---
N = int(input("Enter number of queens (N): "))
solve_nq(N)