searchBST = function(root, val) {

  if(root===null){
    return root
  }
  if(root.val === val){
    return root
  }else if(root.val>val){
    return searchBST(root.left,val)
  }else if(root.val<val){
    return searchBST(root.right,val)
  }
}
@param {TreeNode} root
@param {number} val
@return {TreeNode}