lowestCommonAncestor = function(root, p, q) {
  if(root==null){
    return null
  }
  if(root==p || root==q){
    return root
  }
  let left = lowestCommonAncestor(root.left,p,q)
  let right = lowestCommonAncestor(root.right,p,q)
  if(left && right){
    return root
  }
  return left?left:right
}
@param {TreeNode} root
@param {TreeNode} p
@param {TreeNode} q
@return {TreeNode}