Maximum Depth of Binary Tree: the idea to remember.

The core insight

Depth is one plus the larger child depth; an empty tree has depth zero.

Why it works

A longest path from a nonempty root must continue through its deeper child.

The mistake to avoid

The common convention counts nodes. An empty tree is zero and a leaf is one.

Commented Java solution

Use java.util.* imports and a surrounding solution class. Tree snippets assume the standard TreeNode definition; linked lists use ListNode and Clone Graph uses a Node with val and neighbors; Copy Random List uses Node with val, next and random. Read the input assumptions above.

int maxDepth(TreeNode root) {
    if (root == null) return 0;
    return 1 + Math.max(maxDepth(root.left), maxDepth(root.right));
}

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