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0102-binary-tree-level-order-traversal/0102-binary-tree-level-order-traversal.py

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+# Definition for a binary tree node.
+# class TreeNode:
+#     def __init__(self, val=0, left=None, right=None):
+#         self.val = val
+#         self.left = left
+#         self.right = right
+class Solution:
+    def levelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
+        if root is None:
+            return []
+
+        outNodes = [[root]]
+        out = [[root.val]]
+        nextHasChildren = root.left is not None or root.right is not None
+
+        while nextHasChildren:
+            nextHasChildren = False
+            nextLevel = []
+            nextLevelNodes = []
+
+            for node in outNodes[-1]:
+                if node.left is not None:
+                    nextLevel.append(node.left.val)
+                    nextLevelNodes.append(node.left)
+                    nextHasChildren |= node.left.left is not None or node.left.right is not None
+
+                if node.right is not None:
+                    nextLevel.append(node.right.val)
+                    nextLevelNodes.append(node.right)
+                    nextHasChildren |= node.right.left is not None or node.right.right is not None
+            out.append(nextLevel)
+            outNodes.append(nextLevelNodes)
+
+        return out
+