Leetcode #1245: Tree Diameter
In this guide, we solve Leetcode #1245 Tree Diameter in Python and focus on the core idea that makes the solution efficient.
You will see the intuition, the step-by-step method, and a clean Python implementation you can use in interviews.
Problem Statement
The diameter of a tree is the number of edges in the longest path in that tree. There is an undirected tree of n nodes labeled from 0 to n - 1.
Quick Facts
- Difficulty: Medium
- Premium: Yes
- Tags: Tree, Depth-First Search, Breadth-First Search, Graph, Topological Sort
Intuition
The data forms a graph, so we should explore nodes and edges systematically.
A traversal ensures we visit each node once while maintaining the needed state.
Approach
Build an adjacency list and traverse with BFS or DFS.
Aggregate results as you visit nodes.
Steps:
- Build the graph.
- Traverse with BFS/DFS.
- Accumulate the required output.
Example
Input: edges = [[0,1],[0,2]]
Output: 2
Explanation: The longest path of the tree is the path 1 - 0 - 2.
Python Solution
class Solution:
def treeDiameter(self, edges: List[List[int]]) -> int:
def dfs(i: int, fa: int, t: int):
for j in g[i]:
if j != fa:
dfs(j, i, t + 1)
nonlocal ans, a
if ans < t:
ans = t
a = i
g = defaultdict(list)
for a, b in edges:
g[a].append(b)
g[b].append(a)
ans = a = 0
dfs(0, -1, 0)
dfs(a, -1, 0)
return ans
Complexity
The time complexity is O(V+E). The space complexity is O(V).
Edge Cases and Pitfalls
Watch for boundary values, empty inputs, and duplicate values where applicable. If the problem involves ordering or constraints, confirm the invariant is preserved at every step.
Summary
This Python solution focuses on the essential structure of the problem and keeps the implementation interview-friendly while meeting the constraints.