114. Flatten Binary Tree to Linked List

Problem:

Given a binary tree, flatten it to a linked list in-place.

For example, given the following tree:

    1
   / \
  2   5
 / \   \
3   4   6

The flattened tree should look like:

1
 \
  2
   \
    3
     \
      4
       \
        5
         \
          6

Solution:

Return the leaf node of a flattened subtree for concatenation.

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

/**
 * @param {TreeNode} root
 * @return {TreeNode} leaf node of a flattened subtree
 */
function _flatten (root) {
  if (!root) { return null }
  const leftLeaf = _flatten(root.left)
  const rightLeaf = _flatten(root.right)
  if (leftLeaf !== null) {
    leftLeaf.right = root.right
    root.right = root.left
  } else if (rightLeaf === null) {
    return root
  }
  
  root.left = null
  return rightLeaf || leftLeaf
}