n-queen.py

global N 
N = 4
  
def printSolution(board): 
    for i in range(N): 
        for j in range(N): 
            print board[i][j], 
        print

def isSafe(board, row, col): 
  
    # 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 solveNQUtil(board, col): 
    # base case: If all queens are placed 
    # then return true 
    if col >= N: 
        return True
  
    for i in range(N): 
  
        if isSafe(board, i, col): 
            # Place this queen in board[i][col] 
            board[i][col] = 1
  
            # recur to place rest of the queens 
            if solveNQUtil(board, col + 1) == True: 
                return True
 
            # queen from board[i][col] 
            board[i][col] = 0
    return False
  
def solveNQ(): 
    board = [ [0, 0, 0, 0], 
              [0, 0, 0, 0], 
              [0, 0, 0, 0], 
              [0, 0, 0, 0] 
             ] 
  
    if solveNQUtil(board, 0) == False: 
        print "Solution does not exist"
        return False
  
    printSolution(board) 
    return True
  
# driver program to test above function 
solveNQ()