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# Sink even nodes in Binary Tree

• Difficulty Level : Medium
• Last Updated : 01 Sep, 2021

Given a Binary Tree having odd and even elements, sink all its even valued nodes such that no node with an even value could be a parent of a node with an odd value.

There can be multiple outputs for a given tree, we need to print one of them. It is always possible to convert a tree (Note that a node with odd nodes and all even nodes follow the rule)

Examples:

```Input:
1
/    \
5       8
/  \     /  \
2    4   9    10
Output:
1
5 9
2 4 8 10

Level order traversal after
sinking all the nodes

Input:
4
/  \
2    1
Output:
4
2 1```
```Explanation:
In the first case
Given tree
4
/    \
2      1

There are two trees possible
1            1
/    \   OR   /   \
2      4      4     2

In the second example also,
Given tree
1
/    \
5       8
/  \     /  \
2    4   9    10

There are more than one tree
that can satisfy the condition
1                 1
/    \            /    \
5       9    OR   5      9
/  \    /  \      /  \   / \
2   4  8   10    4    2  8  10```

Approach:

• Basically, it is required to swap the even value of a node with the odd value of one of its descendants.
• The idea is to traverse the tree in a postorder fashion.
• Since we process in postorder, for each even node encountered, its left and right subtrees are already balanced (sinked).
• Check if it’s an even node and its left or right child has an odd value. If the odd value is found, swap the node’s data with that of the odd child node and call the procedure on the odd child to balance the subtree.
• If both children have even values, that means that all their descendants are even.

Below is the implementation of the idea:

## Python

 `# Python3 program to sink even nodes``# to the bottom of binary tree` `# A binary tree node``# Helper function to allocates a new node``class` `newnode:` `    ``# Constructor to create a new node``    ``def` `__init__(``self``, key):``        ``self``.data ``=` `key``        ``self``.left ``=` `None``        ``self``.right ``=` `None` `# Helper function to check``# if node is leaf node``def` `isLeaf(root):``    ``return` `(root.left ``=``=` `None` `and``            ``root.right ``=``=` `None``)` `# A recursive method to sink a tree with even root``# This method assumes that the subtrees are``# already sinked. This method is similar to``# Heapify of Heap-Sort``def` `sink(root):``    ` `    ``# If None or is a leaf, do nothing``    ``if` `(root ``=``=` `None` `or` `isLeaf(root)):``        ``return``    ` `    ``# if left subtree exists and``    ``# left child is even``    ``if` `(root.left ``and` `(root.left.data & ``1``)):``        ` `        ``# swap root's data with left child``        ``# and fix left subtree``        ``root.data, root.left.data ``=` `root.left.data, root.data``        ``sink(root.left)``        ` `    ``# if right subtree exists and``    ``# right child is even``    ``elif``(root.right ``and` `(root.right.data & ``1``)):``        ` `        ``# swap root's data with right child``        ``# and fix right subtree``        ``root.data, root.right.data ``=` `root.right.data, root.data``        ``sink(root.right)` `# Function to sink all even nodes to``# the bottom of binary tree. It does``# a postorder traversal and calls sink()``# if any even node is found``def` `sinkevenNodes(root):``    ` `    ``# If None or is a leaf, do nothing``    ``if` `(root ``=``=` `None` `or` `isLeaf(root)):``        ``return``        ` `    ``# Process left and right subtrees``    ``# before this node``    ``sinkevenNodes(root.left)``    ``sinkevenNodes(root.right)``    ` `    ``# If root is even, sink it``    ``if` `not` `(root.data & ``1``):``        ``sink(root)` `# Helper function to do Level Order Traversal``# of Binary Tree level by level. This function``# is used here only for showing modified tree.``def` `printLevelOrder(root):``    ``q ``=` `[]``    ``q.append(root)``    ` `    ``# Do Level order traversal``    ``while` `(``len``(q)):``        ` `        ``nodeCount ``=` `len``(q)``        ` `        ``# Print one level at a time``        ``while` `(nodeCount):``            ``node ``=` `q[``0``]``            ``print``(node.data, end ``=` `" "``)``            ``q.pop(``0``)``            ``if` `(node.left !``=` `None``):``                ``q.append(node.left)``            ``if` `(node.right !``=` `None``):``                ``q.append(node.right)``            ``nodeCount ``-``=` `1``        ` `        ``# Line separator for levels``        ``print``()` `# Driver Code``""" Constructed binary tree is``            ``1``        ``/ \``        ``5 8``        ``/ \ / \``    ``2 4 9 10     """``root ``=` `newnode(``1``)``root.left ``=` `newnode(``5``)``root.right ``=` `newnode(``8``)``root.left.left ``=` `newnode(``2``)``root.left.right ``=` `newnode(``4``)``root.right.left ``=` `newnode(``9``)``root.right.right ``=` `newnode(``10``)` `sinkevenNodes(root)` `printLevelOrder(root)` `# This code is contributed by SHUBHAMSINGH10`

## Javascript

 ``
Output:
```1
5 9
2 4 8 10```

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