How to Solve Postorder Traversal on LeetCode
Postorder Traversal is a core binary tree problem and is essential for tree algorithm mastery.
Problem Statement
Given the root of a binary tree, return the postorder traversal of its nodes' values. Postorder: left, right, root.
Example:
Input: root = [1,null,2,3]
Output: [3,2,1]
Why This Problem Matters for Interviews
- Tests recursion and stack fundamentals
- Helps build intuition for tree-based algorithms
- Foundation for tree modification tasks
Approach to Postorder Traversal
Recursive DFS (Optimal)
Traverse left, then right, then process the root.
Time Complexity: O(n)
Space Complexity: O(n) (call stack)
def postorderTraversal(root):
res = []
def dfs(node):
if node:
dfs(node.left)
dfs(node.right)
res.append(node.val)
dfs(root)
return res
Iterative with Stack (Alternative)
Use a stack to simulate the call stack.
def postorderTraversal(root):
if not root:
return []
stack, res = [root], []
while stack:
node = stack.pop()
res.append(node.val)
if node.left:
stack.append(node.left)
if node.right:
stack.append(node.right)
return res[::-1]
Key Interview Talking Points
- Postorder order: left, right, root
- Recursive vs iterative solutions
- Stack reversal in iterative approach
Big O Complexity Recap
| Approach | Time Complexity | Space Complexity |
|---|---|---|
| Recursive | O(n) | O(n) |
| Iterative | O(n) | O(n) |
Pro Interview Tips
- Draw tree diagrams with order annotations
- Explain recursion stack behavior
- Know in-place traversal variants (Morris, advanced)