mergeTrees = function(root1, root2) {

  function dfs(root1,root2){
    if(!root1){
      return root2
    }
    if(!root2){
      return root1
    }
    root1.val += root2.val
    root1.left =dfs(root1.left,root2.left)
    root1.right =dfs(root1.right,root2.right)
    return root1
  }
  return dfs(root1,root2)
}
@param {TreeNode} root1
@param {TreeNode} root2
@return {TreeNode}