Binary Tree Preorder Traversal in Python

Suppose we have a binary tree. We have to return the preorder traversal of that tree. So if the tree is like −

Then the preorder traversal will be: [3,9,20,15,7]

To solve this, we will follow these steps −

• make empty lists called res and st.
• node := root
• while node or st is not empty
  • while node is not null, then
    • insert val of node into res, insert node into st and set node := left of node
  • temp := last element of st, and delete last element of st
  • if right of temp is available, then
    • node := right of temp
• 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.data = data       self.left = left       self.right = right def insert(temp,data):    que = []    que.append(temp)    while (len(que)):       temp = que[0]       que.pop(0)       if (not temp.left):          temp.left = TreeNode(data)          break       else:          que.append(temp.left)       if (not temp.right):          temp.right = TreeNode(data)          break       else:          que.append(temp.right) def make_tree(elements):    Tree = TreeNode(elements[0])    for element in elements[1:]:    insert(Tree, element)    return Tree class Solution(object):    def preorderTraversal(self, root):       res = []       st = []       node = root       while node or st:          while node:             if node.data != None:                res.append(node.data)             st.append(node)             node = node.left          temp = st[-1]          st.pop()          if temp.right:             node = temp.right    return res ob1 = Solution() head = make_tree([3,9,20,None,None,15,7]) print(ob1.preorderTraversal(head))

Input

[3,9,20,null,null,15,7]

Output

[3, 9, 20, 15, 7]