【java】查找树按序转双向链表

1.把二元查找树转变成排序的双向链表

题目：

输入一棵二元查找树，将该二元查找树转换成一个排序的双向链表。

要求不能创建任何新的结点，只调整指针的指向。

10

/ \

6 14

/ \ / \

4 8 12 16

转换成双向链表

4=6=8=10=12=14=16

解答：中序遍历，转为双向链表

public ListNode linkList(BSTreeNode root, ListNode head, ListNode pre){
if(root != null) {
if (root.left != null) {
head = linkList(root.left, head, pre);
}
pre = head;
ListNode temp = new ListNode(root.val);
head.next = temp;
head = temp;
head.pre = pre;
if (root.right != null) {
head = linkList(root.right, head, pre);
}
}
return head;

测试：

public static class BSTreeNode{
int val = 0;
BSTreeNode left;
BSTreeNode right;
public BSTreeNode(int value){
val = value;
}
}
public  static class ListNode{
int val;
ListNode pre;
ListNode next;
public ListNode(int value){
val = value;
}
}
public static void main(String[] agrs){
BSTreeNode root = new BSTreeNode(10);
BSTreeNode left = new BSTreeNode(6);
BSTreeNode right = new BSTreeNode(12);
BSTreeNode left1 = new BSTreeNode(4);
BSTreeNode right1 = new BSTreeNode(8);
root.left = left;
root.right = right;
left.left = left1;
left.right = right1;
ListNode head = new ListNode(0);
ListNode pre = new ListNode(0);
algorithm al = new algorithm();
al.linkList(root, head, pre);
while(head != null){
System.out.println(head.val);
head = head.next;
}
}