From 74eb49cf5544cc428e6595360a412337d6a6ee46 Mon Sep 17 00:00:00 2001
From: Piyush Kumar <51694158+piyush2040@users.noreply.github.com>
Date: Mon, 18 Oct 2021 21:58:51 +0530
Subject: [PATCH] Create 51.n-queen.py

---
 Python/51.n-queen.py | 46 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 46 insertions(+)
 create mode 100644 Python/51.n-queen.py

diff --git a/Python/51.n-queen.py b/Python/51.n-queen.py
new file mode 100644
index 0000000..592ba3a
--- /dev/null
+++ b/Python/51.n-queen.py
@@ -0,0 +1,46 @@
+# Link to problem:  https://leetcode.com/problems/n-queens/
+
+# Problem :The n-queens puzzle is the problem of placing n queens on an n x n chessboard such that no two queens attack each other.
+
+#Given an integer n, return all distinct solutions to the n-queens puzzle. You may return the answer in any order.
+
+#Each solution contains a distinct board configuration of the n-queens' placement, where 'Q' and '.' both indicate a queen and an empty space, respectively.
+#Disclaimer : This might not work in your system as input was handled by leetcode only, you can run this code as it is on the problem link
+class Solution:
+    def calc(self,cur_row,col,diag1,diag2,matrix,n):
+        for i in range(n):
+            if col[i]:
+                if cur_row-i not in diag1:
+                    if cur_row+i not in diag2:
+                        matrix[cur_row][i] = 'Q'
+                        if cur_row == n-1:
+                            temp = []
+                            for j in matrix:
+                                #print(j)
+                                temp.append(''.join(j))
+                                #print(temp)
+                            self.ans.append(temp[:])
+                            matrix[cur_row][i] = '.'
+                            temp.clear()
+                            continue
+                        col[i] = False
+                        diag1.add(cur_row-i)
+                        diag2.add(cur_row+i)
+                        self.calc(cur_row+1,col,diag1,diag2,matrix,n)
+                        col[i] = True
+                        diag1.discard(cur_row-i)
+                        diag2.discard(cur_row+i)
+                        matrix[cur_row][i] = '.'
+    
+    
+    
+    def solveNQueens(self, n: int) -> List[List[str]]:
+        self.ans = []
+        col = [True for _ in range(n)]
+        diag1 = set()
+        diag2 = set()
+        matrix = [['.' for i in range(n)] for _ in range(n)]
+        self.calc(0,col,diag1,diag2,matrix,n)
+        return self.ans
+#Input: n = 4
+#Output: [[".Q..","...Q","Q...","..Q."],["..Q.","Q...","...Q",".Q.."]]