二叉搜索樹轉化爲雙向鏈表

題目描述：
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。

思路一：
按中序遍歷，藉助中間隊列用於存儲樹種的各個節點，然後遍歷隊列，給相鄰節點建立關係；

思路二：
不引入新得變量。只能採用遞歸的方式：
假設root節點的左右子樹已經是雙向鏈表，那麼只用將root和左右子樹對應鏈表的最大值和最小值建立連接；

示例代碼：

public class Main {
    public static void main(String args[]) {
        TreeNode node1 = new TreeNode(1), node2 = new TreeNode(2), node3 = new TreeNode(3), node4 = new TreeNode(4);
        node2.left = node1;
        node2.right = node3;
        node3.right = node4;
        TreeNode head = Convert(node2);
        while (head != null) {
            System.out.print(head.val + "-->");
            head = head.right;
        }
    }

    public static TreeNode Convert(TreeNode r) {
        if (r == null)
            return null;
        if (r.left != null) {
            r.left = midTraverseLeft(r.left);
            r.left.right = r;
        }
        if (r.right != null) {
            r.right = midTraverseRight(r.right);
            r.right.left = r;
        }
        while (r.left != null)
            r = r.left;
        return r;
    }

    public static TreeNode midTraverseLeft(TreeNode r) {
        if (r == null)
            return null;

        if (r.left != null)
            midTraverseLeft(r.left).right = r;

        if (r.right != null)
            midTraverseRight(r.right).left = r;

        if (r.right != null)
            return r.right;

        return r;

    }

    public static TreeNode midTraverseRight(TreeNode r) {
        if (r == null)
            return null;

        if (r.left != null)
            midTraverseLeft(r.left).right = r;

        if (r.right != null)
            midTraverseRight(r.right).left = r;

        if (r.left != null)
            return r.left;

        return r;
    }
}

輸出結果：

1-->2-->3-->4-->

遞歸改進：
從上述代碼中可以看到，代碼相似度很高，可以減少代碼量（但是源代碼更容易理解）。每個root和root.right的情形一模一樣，與root.left不一樣。root要返回最小值（最左邊的節點），root.left要返回最大值（最右邊的節點）。因此，midTraverseRight可以用Convert代替。

代碼如下：

public class Main {
    public static void main(String args[]) {
        TreeNode node1 = new TreeNode(1), node2 = new TreeNode(2), node3 = new TreeNode(3), node4 = new TreeNode(4);
        node2.left = node1;
        node2.right = node3;
        node3.right = node4;
        TreeNode head = Convert(node2);

        while (head != null) {
            System.out.print(head.val + "-->");
            head = head.right;
        }
    }

    public static TreeNode Convert(TreeNode r) {
        if (r == null)
            return null;
        if (r.left != null) {
            r.left = midTraverseLeft(r.left);
            r.left.right = r;
        }
        if (r.right != null) {
            r.right = Convert(r.right);
            r.right.left = r;
        }
        while (r.left != null)
            r = r.left;
        return r;
    }

    public static TreeNode midTraverseLeft(TreeNode r) {
        if (r == null)
            return null;

        if (r.left != null)
            midTraverseLeft(r.left).right = r;

        if (r.right != null)
            Convert(r.right).left = r;

        if (r.right != null)
            return r.right;

        return r;

    }
}

輸出結果：

1-->2-->3-->4-->