Create N-Queens

LeetCode/Problems/Python/N-Queens

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+class Solution:
+    def solveNQueens(self, n: int) -> List[List[str]]:
+
+        if n == 0: return []
+
+        col = set()
+        diagonal = set()    # determined by r+c
+        antidiagonal = set() #
+        output = []
+        result = []
+
+        def backtrack(r):
+            nonlocal n,col,diagonal,antidiagonal,output,result
+            if r == n:
+                result.append(output[:])
+                return
+
+            for c in range(n):
+                if c in col or (r+c) in diagonal or (r-c) in antidiagonal: continue
+
+                col.add(c)
+                diagonal.add(r+c)
+                antidiagonal.add(r-c)
+                output.append('.'*c + 'Q' + '.'*(n-c-1))
+                backtrack(r+1)
+
+                col.remove(c)
+                diagonal.remove(r+c)
+                antidiagonal.remove(r-c)
+                output.pop()
+
+        backtrack(0)
+        return result