merge_binary_trees.js

/*
* 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}
 */

// * Time Complexity: O(n + m);
// * Space Complexity: O(n + m); 

var mergeTrees = function(t1, t2) {
  if(!t1) return t2;
  if(!t2) return t1;
    
  let newTree = new TreeNode(t1.val + t2.val);

  if(t1.left || t2.left){
    newTree.left = mergeTrees(t1.left, t2.left);
  }

  if(t1.right || t2.right){
    newTree.right = mergeTrees(t1.right, t2.right);
  }
  
  return newTree;
};