0617-merge-two-binary-trees.js

/**
 * 617. Merge Two Binary Trees
 * https://leetcode.com/problems/merge-two-binary-trees/
 * Difficulty: Easy
 *
 * Given two binary trees and imagine that when you put one
 * of them to cover the other, some nodes of the two trees
 * are overlapped while the others are not.
 *
 * You need to merge them into a new binary tree.
 * The merge rule is that if two nodes overlap, then sum
 * node values up as the new value of the merged node.
 * Otherwise, the NOT null node will be used as the
 * node of new tree.
 */

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} t1
 * @param {TreeNode} t2
 * @return {TreeNode}
 */
var mergeTrees = function(t1, t2) {
  if (!t1) return t2;
  if (!t2) return t1;

  t1.val += t2.val;
  t1.left = mergeTrees(t1.left, t2.left);
  t1.right = mergeTrees(t1.right, t2.right);

  return t1;
};