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

 Live Demo

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]