Kth Smallest Element in a BST

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Problem

Given the root of a binary search tree, and an integer k, return the kth smallest value (1-indexed) of all the values of the nodes in the tree.

Example 1:

Input: root = [3,1,4,null,2], k = 1
Output: 1

Example 2:

Input: root = [5,3,6,2,4,null,null,1], k = 3
Output: 3

Pseudocode

- leverage on behaviour of dfs
    - recurse to end of left child node
    - then return the kth node

Solution

// BST - node.right < node < node.left
// kth smallest - need to find the smallest, then find the kth smallest on the way back
var kthSmallest = function (root, k) {
  let count = 0;
  let smallestK = null;

  function walk(node) {
    // base condition
    if (!node) {
      return 0;
    }

    // pre
    // recurse
    walk(node.left);
    count++;
    // console.log(`smallest ${count}: ${node.val}`)
    if (count === k) {
      smallestK = node.val;
    }
    walk(node.right);
    // post
  }

  walk(root);

  return smallestK;
};

Time and Space Complexity

Time

• What did the code do

• Total -

Space

• What did the code do

• Total -