劍指offer：將二叉搜索樹轉變成雙向鏈表

輸入一棵二叉搜索樹，將該二叉搜索樹轉換成一個排序的雙向鏈表。要求不能創建任何新的結點，只能調整樹中結點指針的指向。

思路：手動寫一顆二叉搜索樹，發現中序遍歷的結果是有序的，那麼將中序遍歷的結果保存起來，然後遍歷結果集合， 更改樹節點的指向即可。

import java.util.LinkedList;

public class Convert {
    public static void main(String[] args) {
        TreeNode root = new TreeNode(10);
        TreeNode treeNode2 = new TreeNode(5);
        TreeNode treeNode3 = new TreeNode(7);
        TreeNode treeNode4 = new TreeNode(12);
        root.left = treeNode2;
        root.right = treeNode4;
        treeNode2.right = treeNode3;

        Convert convert = new Convert();
        LinkedList<TreeNode> linkedList = new LinkedList<>();
        TreeNode head = convert.Convert(root);
        while(head != null) {
            System.out.println(head.val);
            head = head.right;
        }
    }

    public TreeNode Convert(TreeNode pRootOfTree) {
        LinkedList<TreeNode> linkedList = new LinkedList<>();
        midLook(linkedList, pRootOfTree);
        TreeNode pre = null;
        TreeNode cur = null;
        TreeNode head = linkedList.peek();
        while (!linkedList.isEmpty()) {
            cur = linkedList.pop();
            cur.left = pre;
            cur.right = linkedList.isEmpty() ? null : linkedList.peek();
            pre = cur;
        }

        return head;
    }

    public void midLook(LinkedList<TreeNode> linkedList, TreeNode root) {
        if (root == null) {
            return;
        }
        midLook(linkedList, root.left);
        linkedList.add(root);
        midLook(linkedList, root.right);
    }

}