Leetcode: Binary tree inorder traversal
The task is to traverse a binary tree inorder. This means, that starting from the root node, nodes should be visited in the following order: left, self, right. With this knowledge it is easy to come up with a recursive algorithm.
The only special case that needs to be handled are the leaf nodes that don’t have any children. By still visiting the non-existing children and then returning an empty list ([]) when a None is encountered, the whole algorithm fits into two lines.
class TreeNode:
def __init__(self, val=0, left=None, right=None):
self.val = val
self.left = left
self.right = right
class Solution:
def inorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
# Handle non-existing leaf-nodes (including empty root)
if root is None:
return []
# Visit left - current - right
return self.inorderTraversal(root.left) + [root.val] + self.inorderTraversal(root.right)
if __name__ == '__main__':
s = Solution().inorderTraversal
print(s(TreeNode(1, None, TreeNode(2, TreeNode(3), None))), [1,3,2])