<p>Given the <code>root</code> of a binary search tree (BST) with duplicates, return <em>all the <a href="https://en.wikipedia.org/wiki/Mode_(statistics)" target="_blank">mode(s)</a> (i.e., the most frequently occurred element) in it</em>.</p> <p>If the tree has more than one mode, return them in <strong>any order</strong>.</p> <p>Assume a BST is defined as follows:</p> <ul> <li>The left subtree of a node contains only nodes with keys <strong>less than or equal to</strong> the node's key.</li> <li>The right subtree of a node contains only nodes with keys <strong>greater than or equal to</strong> the node's key.</li> <li>Both the left and right subtrees must also be binary search trees.</li> </ul> <p> </p> <p><strong class="example">Example 1:</strong></p> <img alt="" src="https://assets.leetcode.com/uploads/2021/03/11/mode-tree.jpg" style="width: 142px; height: 222px;" /> <pre> <strong>Input:</strong> root = [1,null,2,2] <strong>Output:</strong> [2] </pre> <p><strong class="example">Example 2:</strong></p> <pre> <strong>Input:</strong> root = [0] <strong>Output:</strong> [0] </pre> <p> </p> <p><strong>Constraints:</strong></p> <ul> <li>The number of nodes in the tree is in the range <code>[1, 10<sup>4</sup>]</code>.</li> <li><code>-10<sup>5</sup> <= Node.val <= 10<sup>5</sup></code></li> </ul> <p> </p> <strong>Follow up:</strong> Could you do that without using any extra space? (Assume that the implicit stack space incurred due to recursion does not count).