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

/**
 * 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) {
  helper(root);
};

var helper = function (root) {
  if (!root) return null;

  var leftLast = helper(root.left);
  var rightLast = helper(root.right);

  if (root.left) {
    leftLast.right = root.right;
    root.right = root.left;
  }

  root.left = null;

  return rightLast || leftLast || root;
};

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(1).