Kth Smallest Element in a BST: the idea to remember.

The core insight

Iteratively traverse inorder using a stack and return the value at the kth visited node.

Why it works

Inorder traversal visits a valid BST in increasing order, so visit rank equals sorted rank.

The mistake to avoid

Descending left does not count a visit. Count only when a node is popped for processing.

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 kthSmallest(TreeNode root, int k) {
    Deque<TreeNode> stack = new ArrayDeque<>(); TreeNode current = root;
    while (current != null || !stack.isEmpty()) {
        while (current != null) { stack.push(current); current = current.left; }
        current = stack.pop();
        if (--k == 0) return current.val;
        current = current.right;
    }
    throw new IllegalArgumentException("k is outside the tree");
}

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