Verify Preorder Sequence in Binary Search Tree — LeetCode 255 Python Solution
MediumLeetCode PremiumStackTreeBinary Search TreeRecursionArrayBinary TreeMonotonic Stack
- Problem
- #255
- Pattern
- Stack
- Reading time
- 2 min
- Source
- leetcode.com
The problem
Given an array of unique integers preorder, return true if it is the correct preorder traversal sequence of a binary search tree.
Example
- Input
- preorder = [5,2,1,3,6]
- Output
- true
Python solution
Python
class Solution:
def verifyPreorder(self, preorder: List[int]) -> bool:
stk = []
last = -inf
for x in preorder:
if x < last:
return False
while stk and stk[-1] < x:
last = stk.pop()
stk.append(x)
return TrueComplexity
| Measure | Complexity |
|---|---|
| Time | O(n) |
| Space | O(n) auxiliary |
Pattern: Stack
When the most recent unresolved thing is the one that matters, use a stack. LeetCode 255. Verify Preorder Sequence in Binary Search Tree is filed here because LeetCode tags it Stack, which is the vocabulary this hub collects.
The stack guide has the Python template for the pattern and the 194 LeetCode problems that use it.
Related problems
Frequently asked questions
- How hard is LeetCode 255. Verify Preorder Sequence in Binary Search Tree?
- LeetCode 255. Verify Preorder Sequence in Binary Search Tree is rated Medium on LeetCode.
- What is the time complexity of LeetCode 255. Verify Preorder Sequence in Binary Search Tree?
- The Python solution on this page runs in O(n).
- What is the space complexity of LeetCode 255. Verify Preorder Sequence in Binary Search Tree?
- The Python solution on this page uses O(n) auxiliary space.
- What topics does LeetCode 255. Verify Preorder Sequence in Binary Search Tree cover?
- LeetCode 255. Verify Preorder Sequence in Binary Search Tree is tagged Stack, Tree, Binary Search Tree, Recursion, Array, Binary Tree and Monotonic Stack on LeetCode.
- Is LeetCode 255. Verify Preorder Sequence in Binary Search Tree a premium problem?
- Yes. LeetCode 255. Verify Preorder Sequence in Binary Search Tree is a LeetCode Premium problem, so the full statement and test cases require a paid LeetCode subscription.