PHP implements pre-order/in-order/post-order traversal binary tree operations based on non-recursive algorithm

/** * PHP基于非递归方式算法实现先序/中序/后序遍历二叉树操作 * A * B C * D E F G * H * 先序遍历:先遍历根节点,然后遍历左节点,最后遍历右节点: ABDHECFG * 中序遍历:先遍历左子树,然后遍历根节点,最后遍历右子树: HDBEAFCG * 后序遍历:先遍历左子树,然后遍历右子树,最后遍历根节点: HDEBFGCA * */ /*先序遍历:利用栈的先进后出特性,先访问根节点,再把右子树压入,再压入左子树.这样取出的时候是先取出左子树,最后取出右子树*/ function preOrder($root){ $stack = array(); array_push($stack,$root); while(!empty($stack)){ $center_node = array_pop($stack); echo $center_node->value;//根节点 if($center_node->right != null){ array_push($stack,$center_node->right);//压入右子树 } if($center_node->left != null){ array_push($stack,$center_node->left);//压入左子树 } } } /*中序遍历:需要从下向上遍历,所以先把左子树压入栈,然后逐个访问根节点和右子树*/ function inOrder($root){ $stack = array(); $center_node = $root; while(!empty($stack) || $center_node != null){ while($center_node != null ){ array_push($stack,$center_node); $center_node = $center_node->left; } $center_node = array_pop($stack); echo $center_node->value; $center_node = $center_node->right; } } /*后序遍历:先把根节点存起来,然后依次存储左子树和右子树,然后输出*/ function tailOrder($root){ $stack = array(); $outStack = array(); array_push($$stack, $root); while($empty($stack)){ $center_node = array_pop($stack); array_push($outStack,$center_node); if($center_node->right != null){ array_push($stack,$center_node->left); } } while($empty($outStack)){ echo $center_node->value; } }