What is the Tree DFS pattern?

The Tree DFS pattern is a systematic approach to solving a class of coding interview problems. By recognizing the Tree DFS structure, you can solve problems in optimal time complexity using proven techniques.

When should I use the Tree DFS pattern?

Use the Tree DFS pattern when the problem structure matches its recognition signals — typically when brute force would be O(n²) or worse, and the pattern reduces complexity to O(n).

What is the time complexity of Tree DFS?

The Tree DFS pattern typically runs in O(n) time and uses O(h) where h is height space. The exact complexity depends on the specific sub-variant being applied.

Is Tree DFS important for coding interviews?

Yes — the Tree DFS pattern is frequently tested at FAANG companies and competitive engineering interviews. Mastering this pattern gives you a reliable framework to solve many problem variations quickly.

How do I practice the Tree DFS pattern?

Start by studying the pattern's mental model and recognition signals on DSAPatterns, then practice the curated LeetCode problems below in order from Easy to Hard. Aim for pattern recognition before memorizing solutions.

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pattern / MEDIUM

Tree DFS.

Depth-first traversal of trees

complexity

time

O(n)

space

O(h) where h is height

practice questions

browse and solve all 0 pattern questions →

02' — visual

interactive

Sliding window — k = 3

Maximum sum subarray

Window

[0, 2]

Sum

Step

0 / 3

03'

mental
model.

Recursively visit children before processing current node (postorder), or process current node before children (preorder).

Tree DFS patterns use recursion or stack to traverse trees depth-first. Variants include preorder, inorder, postorder, and path-based traversals.

when to use ✓

01Tree traversals (preorder, inorder, postorder)
02Maximum depth, diameter
03Path sum problems
04LCA (lowest common ancestor)

when not ✗

01When level-order information is needed (use BFS)
02When shortest path matters (BFS for unweighted)

04'

recognition signals

Tree problems

Need to visit all nodes

Path from root to leaf

Subtree queries

PreorderInorderPostorderPath sumTree DP

05'

templates

python

1# Beginner: Fixed-size sliding window
2def max_sum_k(nums, k):
3   n = len(nums)
4   if n < k:
5      return 0
6   window_sum = sum(nums[:k])
7   max_sum = window_sum
8   for i in range(k, n):
9      window_sum += nums[i] - nums[i - k]
10      max_sum = max(max_sum, window_sum)
11   return max_sum

common mistakes

01Forgetting base case (null node)
02Confusing when to add to result (pre vs post)
03Stack overflow on deep trees

interview tips

01Always think recursively first
02Master all three traversals
03Practice iterative versions with stack

06'

practice this pattern

0 curated problems from LeetCode

See all

No questions found for this pattern yet.

07'

faq

1# Beginner: Fixed-size sliding window

2def max_sum_k(nums, k):

3 n = len(nums)

4 if n < k:

5 return 0

6 window_sum = sum(nums[:k])

7 max_sum = window_sum

8 for i in range(k, n):

9 window_sum += nums[i] - nums[i - k]

10 max_sum = max(max_sum, window_sum)

11 return max_sum