0114-flatten-binary-tree-to-linked-list.js

/**
 * 114. Flatten Binary Tree to Linked List
 * https://leetcode.com/problems/flatten-binary-tree-to-linked-list/
 * Difficulty: Medium
 *
 * Given the root of a binary tree, flatten the tree into a "linked list":
 * - The "linked list" should use the same TreeNode class where the right child pointer points
 *   to the next node in the list and the left child pointer is always null.
 * - The "linked list" should be in the same order as a pre-order traversal of the binary tree.
 */

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {void} Do not return anything, modify root in-place instead.
 */
var flatten = function(root) {
  if (root === null) return;

  if (root.left) {
    let previous = root.left;
    while (previous.right) {
      previous = previous.right;
    }
    const rightNode = root.right;
    root.right = root.left;
    previous.right = rightNode;
    root.left = null;
  }

  flatten(root.right);
};