<?php#将二叉查找树转换为双向链表，要求不能创建新节点，只能调节节点指针#解题思路是从树的底层开始，调整每个节点的左右子树，将左子树的最大节点与根节点相连，将又子树的最小节点与根节点相连#我们把节点的left当做链表的pre指针，right当做链表的next指针#直到所有子树以及根节点转换完成#树节点class Node {public $data = null;public $parent = null;public $left = null;public $right = null;}#查找树最小节点function search_min($root) {$cnode = $root;while ($cnode->left != null) {$cnode = $cnode->left;}return $cnode;}#查找树的最打节点function search_max($root) {$cnode = $root;while ($cnode->right != null) {$cnode = $cnode->right;}return $cnode;}#查找某个节点中序遍历的前趋function predecessor($node) {#如果该节点存在左子树，则查找其左子树的最大值if ($node->left != null) {return search_max($left);}#否则查找其父节点，直到当前节点是其父节点的右孩子$p = $node->parent;while ($p != null && $node == $p->left) {$node = $p;$p = $p->parent;}return $p;}#查找某个节点中序遍历的后继function successor($node) {#如果该节点存在右子树，则查找其右子树的最小值if ($node->right != null) {return search_min($node->right);}#否则查找其父节点，直到当前节点是其父节点的左孩子$p = $node->parent;while ($p != null && $node == $p->right) {$node = $p;$p = $p->parent;}return $p;}#插入节点function insert_node($root, $node) {$cnode = $root;$p = $root;#查找合适的插入位置while ($cnode != null) {$p = $cnode;    if ($cnode->data > $node->data) {$cnode = $cnode->left;} else {$cnode = $cnode->right;}}if ($p == null) { #如果p为null，说明是空树$root = $node;} else {if ($p->data > $node->data) {$p->left = $node;} else {$p->right = $node;}$node->parent = $p;    }return $root;}#删除结点function delete_node($root, $dnode) {if ($dnode->left != null || $dnode->right != null) {$c = $dnode;} else {$c = successor($dnode);}if ($c->left != null) {$s = $c->left;} else {$s = $c->right;}if ($s != null) {$s->parent = $c->parent;}if ($c->parent == null) { #c是根节点$root = $s;} else if ($c == $c->parent->left) {$c->parent->left = $s; } else {$c->parent->right = $s;}if ($dnode != $c) {$dnode->data = $c->data;}return $root;}#使用数组建立二叉查找树function build_btree($a) {$root = new Node();$root->data= $a[0];for ($i = 1; $i < count($a); $i++) {$node = new Node();$node->data = $a[$i];insert_node($root, $node);}return $root;}#二叉树中序遍历function inorder_traverse($root) {if ($root->left != null) {inorder_traverse($root->left);}echo $root->data . " ";if ($root->right != null) {inorder_traverse($root->right);}}#链表的遍历function linked_list_traverse($head) {$node = $head;while ($node != null) {echo $node->data . " ";$node = $node->right;}}#二叉查找树变双向链表function change($root) {#如果有子节点，就往子节点递归，直到最后一个非叶子节点，将他们的左右子节点连接成链表#然后逐层往上递归if ($root->left != null) {btree_to_doublelist($root->left);#这里不用担心调整左右节点以后search_max查找出来的节点会出错，因为虽然调整以后树的结构变了，但是他依然保有二叉树的性质#看图1就知道了$predecessor = search_max($root->left);$predecessor->right = $root;$root->left = $predecessor;}if ($root->right != null) {btree_to_doublelist($root->right);$successor = search_min($root->right);$successor->left = $root;$root->right = $successor;}}function btree_to_doublelist($root) {#链表头节点就是二叉查找树的最小节点$head = search_min($root);change($root);return $head;}$a = array(10, 6, 14, 4, 8, 12, 16);$root = build_btree($a);inorder_traverse($root);echo "<br>";$link_head = btree_to_doublelist($root);linked_list_traverse($link_head);echo "<br>";
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