Added code for assignment B4, i.e. N-Queen problem.

Codes/Assignment-B4.py

@@ -0,0 +1,50 @@
+def placeQueens(i, cols, leftDiagonal, rightDiagonal, cur, board):
+    n = len(cols)
+
+    if i == n:
+        return True
+
+    for j in range(n):
+        if cols[j] or rightDiagonal[i + j] or leftDiagonal[i - j + n - 1]:
+            continue
+
+        cols[j] = 1 
+        rightDiagonal[i + j] = 1
+        leftDiagonal[i - j + n - 1] = 1
+        cur.append(j)
+
+        if placeQueens(i + 1, cols, leftDiagonal, rightDiagonal, cur, board):
+            return True
+
+        cur.pop()
+        cols[j] = 0
+        rightDiagonal[i + j] = 0
+        leftDiagonal[i - j + n - 1] = 0
+
+    return False
+
+def nQueen(n):
+    cols = [0] * n
+    leftDiagonal = [0] * (n * 2)
+    rightDiagonal = [0] * (n * 2)
+    cur = []
+    board = [['.' for _ in range(n)] for _ in range(n)]
+
+    if placeQueens(0, cols, leftDiagonal, rightDiagonal, cur, board):
+        for i in range(n):
+            board[i][cur[i]] = 'Q'
+        return board
+    else:
+        return None
+
+def printBoard(board):
+    if board:
+        for row in board:
+            print(" ".join(row))
+    else:
+        print("No solution exists.")
+
+if __name__ == "__main__":
+    n = int(input("Enter the number of queens:\t"))
+    board = nQueen(n)
+    printBoard(board)

README.md

@@ -15,6 +15,7 @@
 1. [Assignment-A1 (DFS | BFS)](Codes/Assignment-A1.py)
 2. [Assignment-A2 (A star algorithm)]()
 3. [Assignment-A3.I (Selection Sort)](Codes/Assignment-A3.I.py)
+4. [Assignment-A4 (N-Queen Problem)](Codes/Assignment-A4.py)
 
 ### Practical