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Binary Tree Postorder Traversal in Python
Suppose we have a binary tree. We have to find the post order traversal of this tree using the iterative approach. So if the tree is like −
Then the output will be: [9,15,7,10,-10]
To solve this, we will follow these steps −
if root is null, then return empty array
create an array ret
stack := define a stack with pair [root, 0]
while stack is not empty −
node := top of stack, then delete element from stack.
if second value of node pair is 0, then
current := first value of node pair
insert a pair (current, 1) into stack
if right of current is present −
insert a pair [right of current, 0] into stack
if left of current is present −
insert a pair [left of current, 0] into stack
Otherwise when data of the first value node pair is not 0, then insert the data of the first value of node into res
return res
Example
Let us see the following implementation to get a better understanding −
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): if data is not None: temp.left = TreeNode(data) else: temp.left = TreeNode(0) break else: que.append(temp.left) if (not temp.right): if data is not None: temp.right = TreeNode(data) else: temp.right = TreeNode(0) 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 postorderTraversal(self, root): if not root: return [] res = [] stack = [[root,0]] while stack: node = stack[-1] stack.pop() if node[1]== 0 : current = node[0] stack.append([current,1]) if current.right: stack.append([current.right,0]) if current.left: stack.append([current.left,0]) else: if node[0].data != 0: res.append(node[0].data) return res ob = Solution() root = make_tree([-10,9,10,None,None,15,7]) print(ob.postorderTraversal(root))
Input
[-10,9,10,None,None,15,7]
Output
[9, 15, 7, 10, -10]
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