?php
class Node{
  public $value;
  public $leftNode;
  public $rightNode;
}
/* 找到空节点 */
function findEmpytNode($node, $parent = null){
  if(empty($node->value)){
    return $node;
  }else{
    if(empty($node->leftNode->value)){
      return $node->leftNode;
    }else if(empty($node->rightNode->value)){
      return $node->rightNode;
    }else{
      if(empty($parent) || $node->value == $parent->rightNode->value){
        return findEmpytNode($node->leftNode, $node);
      }else{
        return findEmpytNode($parent->rightNode, $node);
      }
    }
  }
}
/* 添加节点 */
function addNode($node, $value){
  $emptyNode = findEmpytNode($node);
  setNode($emptyNode, $value);
}
/* 设置节点 */
function setNode($node, $value){
  $node->value = $value;
  $node->leftNode = new Node();
  $node->rightNode = new Node();
}
/* 打印 */
function printTree($node, $parent = null){
  if(empty($node->value)) return ;
  echo $node->leftNode->value;
  echo $node->rightNode->value;
  if(empty($parent) || $node->value == $parent->rightNode->value){
    printTree($node->leftNode, $node);
  }else{
    printTree($parent->rightNode, $node);
  }
}
$head = new Node();
setNode($head, 1);
addNode($head, 2);
addNode($head, 3);
addNode($head, 4);
addNode($head, 5);
addNode($head, 6);
printTree($head);