<div class="leaf flex"><div class="inner justify"><h4>Notes</h4><p class="  start-chapter">A tree is an undirected and connected acyclic graph.</p><p>Recursion is a common approach for trees. When you notice that the subtree problem can be used to solve the entire problem, try using recursion.</p><p>When using recursion, always remember to check for the base case, usually where the node is <code>null</code>.</p><p>When you are asked to traverse a tree by level, use breadth-first search.</p><p>Sometimes it i