Find center of star graph

Given an undirected star graph's edges, identify the central node connected to all others.

graph star-graph array

A **star graph** has exactly one center node connected to every other node, and there are no other edges. You are given a list of `n - 1` edges where `n` is the number of nodes (numbered from 1 to `n`). Each edge is a pair `[u, v]` meaning there is an undirected edge between node `u` and node `v`. The given graph is guaranteed to be a valid star graph.

Write a function `find_center(edges)` that returns the center node of the star graph.

**Input**: `edges` — a list of integer pairs, each pair `[u, v]` with `1 <= u, v <= 10^5`. The array contains exactly `n - 1` edges for `n >= 3` nodes.

**Output**: an integer — the label of the center node.

**Note**: A straightforward approach comparing the first two edges is sufficient and correct. Do not assume the edges are sorted.

Constraints

3 ≤ n ≤ 10^5 (number of nodes).
edges.length == n - 1.
Each edge is a pair [u, v] with distinct nodes.
Guaranteed to form a valid star graph.
Expected time complexity: O(1) or O(n) — both acceptable; O(1) possible.

Example

>>> find_center([[1,2],[2,3],[4,2]])
2
>>> find_center([[1,2],[5,1],[1,3],[1,4]])
1

10 points ~15 min

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Hints

In a star graph, the center appears in every edge.

Only need to examine the first two edges.

The common node of the first two edges is the center.

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