CSES 1131 Tree Diameter - Double BFS Algorithm

Double BFS finds the diameter in $2$ passes:

$1.$ BFS from any node. Find the farthest node $A$.

$2.$ BFS from $A$. Find the farthest node $B$.

The distance from $A$ to $B$ is the diameter. You do not need to try every starting node. The first BFS always lands on a diameter endpoint, no matter where you start. This runs in $O(n)$ time and uses $O(n)$ space. The Tree Diameter & Center section proves why this property holds and walks through a full example with code.