Count Good Nodes In Binary Tree: the idea to remember.

The core insight

Carry the maximum ancestor value down each root to node path.

Why it works

A node is good exactly when its value reaches or exceeds that path maximum; siblings require independent path state.

The mistake to avoid

Equal values qualify. Negative root values must also be counted.

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.

// Java reference adapted from neetcode-gh/leetcode (MIT).
// Carry the maximum ancestor value down each root to node path.
public int goodNodes(TreeNode root) {
    return helper(root, Integer.MIN_VALUE);
}

public int helper(TreeNode root, int max) {
    if (root == null) return 0;

    int res = root.val >= max ? 1 : 0;

    res += helper(root.left, Math.max(root.val, max));
    res += helper(root.right, Math.max(root.val, max));

    return res;
}

Java reference adapted from neetcode-gh/leetcode · MIT license

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