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Recursion, trees, data structures
Invert a binary tree.
For example, suppose you have the following tree:
This is a classic recursion problem. These types can be identified by manually reviewing several cases. This is usually a good practice as it helps with debugging and resolving edge cases.
This is marked as a medium issue. It’s actually around the lower end of the medium range. Solving such problems requires understanding data structures/processes.
If you look at this example, you can see that the process is basically swapping left and right nodes. If we examine the binary trees themselves, we can see that they are symmetric. There is no difference between the implementation of right and left nodes. A binary tree node is defined as:
Node(val, Node Left, Node Right). Node can be either a node or none. This implementation of self-reference is why recursion works so well with trees.
Back to the problem. How do I flip the tree? Simply swap the node’s two children, then swap the node with its siblings. Use paper and pen and work through small examples to convince yourself that it will work.
def invert(node):if not node:
return node
left = invert(node.left)
right = invert(node.right)
node.left, node.right = right, left
return node
This problem is not that difficult once you know the trick. Many easy medium problems are that way.If you have any questions, please feel free to contact us
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