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Program to traverse binary tree level wise in alternating way in Python
Suppose we have binary tree, we have to show the values of each level by alternating from going left-to-right and right-to-left.
So, if the input is like
then the output will be [5, -10, 4, -2, -7, 15]
To solve this, we will follow these steps −
if root is null, then
return a new list
s1 := a list initially insert root
s2 := a new list
res := a new list
while s1 is not empty or s2 is not empty, do
while s1 is not empty, do
node := delete last element from s1
if left of node is not null, then
insert left of node at the end of s2
if right of node is not null, then
insert right of node at the end of s2
insert value of node at the end of res
while s2 is not empty, do
node := delete last element from s2
if right of node is not null, then
insert right of node at the end of s1
if left of node is not empty, then
insert left of node at the end of s1
insert value of node at the end of res
return res
Let us see the following implementation to get better understanding −
Example
class TreeNode: def __init__(self, data, left = None, right = None): self.val = data self.left = left self.right = right class Solution: def solve(self, root): if not root: return [] s1 = [root] s2 = [] res = [] while s1 or s2: while s1: node = s1.pop() if node.left: s2.append(node.left) if node.right: s2.append(node.right) res.append(node.val) while s2: node = s2.pop() if node.right: s1.append(node.right) if node.left: s1.append(node.left) res.append(node.val) return res ob = Solution() root = TreeNode(5) root.left = TreeNode(4) root.right = TreeNode(-10) root.left.left = TreeNode(-2) root.right.left = TreeNode(-7) root.right.right = TreeNode(15) print(ob.solve(root))
Input
root = TreeNode(5) root.left = TreeNode(4) root.right = TreeNode(-10) root.left.left = TreeNode(-2) root.right.left = TreeNode(-7) root.right.right = TreeNode(15)
Output
[5, -10, 4, -2, -7, 15]
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