Given the root node of a binary search tree and an integer value k, return the kth smallest value from all the nodes of the tree.

kth-smallest-in-BST.js

export function kthSmallestElement(root, k) {
  const items = []
  // traverse tree in order, creating a sorted arrat
  inorder(root, (value) => items.push(value))
  // grab k-1th item as a result
  return items[k - 1];
}

function inorder(root, callback) {
  const stack = [];
  let current = root;
  while (current || stack.length > 0) {
    while (current) {
      stack.push(current)
      current = current.left
    }
    current = stack.pop()
    callback(current.data)
    current = current.right
  }