LeetCode 310 Minimum Height Trees - Implementation

Start with all degree-$1$ nodes (leaves) in a queue. Remove them, decrement neighbors' degrees, add new leaves to queue. Repeat until $\leq 2$ nodes remain. These are the minimum height tree roots. Why $\leq 2$? The centroid of a tree is either one node or an edge. If it's an edge, both endpoints are valid roots. Time: $O(n)$. Each node is added to the queue exactly once. Space: $O(n)$ for the degree array and queue.

Time complexity: $O(n)$.

Space complexity: $O(n)$.