kthSmallest = function(root, k, inOrder = []) {
    if (!root) return inOrder

    return dfs(root, k, inOrder);
}
https://leetcode.com/problems/kth-smallest-element-in-a-bst/
Time O(N + K) | Space O(H)
@param {TreeNode} root
@param {number} k
@return {number}