1. /**
    2.  * Definition for a binary tree node.
    3.  * function TreeNode(val, left, right) {
    4.  *     this.val = (val===undefined ? 0 : val)
    5.  *     this.left = (left===undefined ? null : left)
    6.  *     this.right = (right===undefined ? null : right)
    7.  * }
    8.  */
    9. /**
    10.  * @param {TreeNode} root1
    11.  * @param {TreeNode} root2
    12.  * @return {TreeNode}
    13.  */
    14. var mergeTrees = function(root1, root2) {
    15.     if(!root1) {
    16.         return root2
    17.     }
    19.     if(!root2) {
    20.         return root1
    21.     }
    23.     root1.val = root1.val + root2.val;
    25.     root1.left = mergeTrees(root1.left, root2.left)
    26.     root1.right =  mergeTrees(root1.right, root2.right)
    28.     return root1
    29. };