Univalued Binary Tree
Easy · Trees · Tree · Depth-First Search · Breadth-First Search · Binary Tree
Problem
A binary tree is uni-valued if every node in it holds the same value.
Given the root of a binary tree, return true if the tree is uni-valued and false otherwise.
Examples
Input: root = [4,4,4,null,4] Output: true
All four nodes hold 4.
Input: root = [4,4,4,4,7] Output: false
One leaf holds 7, so the values are not all the same.
Input: root = [2,2,2,5,2] Output: false
The 5 sits two levels down; every node has to be checked, not just the root's children.
Constraints
- The number of nodes is in the range [1, 100]
- 0 <= Node.val < 100
Approaches
- Baseline: collect every value, then count distinct — time O(n), space O(n)
Follow-up
With an early exit the walk stops at the first mismatch, so a tree whose root's children already differ is answered in constant time. The use cases count every uni-valued subtree, which needs answers from both children (a post-order walk), and relax the rule to one value per level.
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