```
/**
* 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&&t2)) return !t1 ? t2 : t1; // !(t1&&t2) could be faster than !t1||!t2
let rst = new TreeNode(t1.val + t2.val);
rst.left = mergeTrees(t1.left , t2.left);
rst.right= mergeTrees(t1.right, t2.right);
return rst;
};
```