Question

如何合并2个二叉搜索树，使得结果树包含两棵树的所有元素，并保持BST属性。

我看到了提供的解决方案 How to merge two BST's efficiently?

然而，该解决方案涉及转换为双重链接列表。我想知道是否有一种更优雅的方式可以在没有转换的情况下完成。我提出了以下伪代码。它适用于所有情况吗？我也遇到了第三种情况的麻烦。

node* merge(node* head1, node* head2) {
        if (!head1)
            return head2;
        if (!head2)
            return head1;

        // Case 1.
        if (head1->info > head2->info) {
            node* temp = head2->right;
            head2->right = NULL;
            head1->left = merge(head1->left, head2);
            head1 = merge(head1, temp);
            return head1;
        } else if (head1->info < head2->info)  { // Case 2
            // Similar to case 1.
        } else { // Case 3
            // ...
        }
}

Answer 1

在递归遍历期间，不能直接合并两个二叉搜索树（BST）。假设我们应该合并图中所示的Tree 1和Tree 2.

Fig1

递归应该将合并减少到更简单的情况。我们不能减少因为L2可以包含，所以仅合并到相应的左子树L1和L2 数字大于10，所以我们需要包含权利子树R1进入过程。但随后我们将数字包括在内超过10，可能超过20，所以我们需要包括右子树R2也是如此。类似的推理表明了这一点我们不能通过包含Tree 1和Tree 2中的子树来简化合并在同一时间。

减少的唯一可能性是仅在相应的树内简化。所以，我们可以改造树木到右边刺有排序节点：

Fig2

现在，我们可以轻松地将两个棘刺合并到一个脊柱中。这个脊柱实际上是一个BST，所以我们可以在这里停下来。但是，这个BST 是完全不平衡的，所以我们将其转换为平衡的BST。

复杂性是：

Spine 1: time = O(n1),    space = O(1) 
Spine 2: time = O(n2),    space = O(1) 
Merge:   time = O(n1+n2), space = O(1) 
Balance: time = O(n1+n2), space = O(1) 
Total:   time = O(n1+n2), space = O(1)

完整的正在运行的代码位于http://ideone.com/RGBFQ。这是必不可少的部分。顶级代码如下：

Node* merge(Node* n1, Node* n2) {
    Node *prev, *head1, *head2;   
    prev = head1 = 0; spine(n1, prev, head1); 
    prev = head2 = 0; spine(n2, prev, head2);
    return balance(mergeSpines(head1, head2));
}

辅助功能用于转变为刺：

void spine(Node *p, Node *& prev, Node *& head) {   
    if (!p) return;   
    spine(p->left, prev, head);   
    if (prev) prev->right = p;   
    else head = p;  
    prev = p; 
    p->left = 0;  
    spine(p->right, prev, head); 
}

刺的合并：

void advance(Node*& last, Node*& n) {
    last->right = n; 
    last = n;
    n = n->right; 
}

Node* mergeSpines(Node* n1, Node* n2) {
    Node head;
    Node* last = &head;
    while (n1 || n2) {
        if (!n1) advance(last, n2);
        else if (!n2) advance(last, n1);
        else if (n1->info < n2->info) advance(last, n1);
        else if (n1->info > n2->info) advance(last, n2);
        else {
            advance(last, n1);
            printf("Duplicate key skipped %d \n", n2->info);
            n2 = n2->right;
        }
    }
    return head.right; 
}

平衡：

Node* balance(Node *& list, int start, int end) {
    if (start > end) return NULL;  
    int mid = start + (end - start) / 2;    
    Node *leftChild = balance(list, start, mid-1);   
    Node *parent = list;
    parent->left = leftChild;   
    list = list->right;   
    parent->right = balance(list, mid+1, end);   
    return parent; 
}   

Node* balance(Node *head) {
    int size = 0;
    for (Node* n = head; n; n = n->right) ++size;
    return balance(head, 0, size-1); 
}

Answer 2

假设我们有两棵树A和B，我们将树A的根插入到树B中并使用旋转移动插入的根以成为树B的新根。接下来，我们递归地合并树A和B的左右子树。该算法考虑了两种树结构，但插入仍取决于目标树的平衡程度。您可以使用此提示在O(n+m)时间和O(1)空间中合并两棵树。

以下实施归因于Dzmitry Huba：

// Converts tree to sorted singly linked list and appends it
// to the head of the existing list and returns new head.
// Left pointers are used as next pointer to form singly
// linked list thus basically forming degenerate tree of
// single left oriented branch. Head of the list points
// to the node with greatest element.
static TreeNode<T> ToSortedList<T>(TreeNode<T> tree, TreeNode<T> head)
{
    if (tree == null)
        // Nothing to convert and append
        return head;
    // Do conversion using in order traversal
    // Convert first left sub-tree and append it to
    // existing list
    head = ToSortedList(tree.Left, head);
    // Append root to the list and use it as new head
    tree.Left = head;
    // Convert right sub-tree and append it to list
    // already containing left sub-tree and root
    return ToSortedList(tree.Right, tree);
}

// Merges two sorted singly linked lists into one and
// calculates the size of merged list. Merged list uses
// right pointers to form singly linked list thus forming
// degenerate tree of single right oriented branch.
// Head points to the node with smallest element.
static TreeNode<T> MergeAsSortedLists<T>(TreeNode<T> left, TreeNode<T> right, IComparer<T> comparer, out int size)
{
    TreeNode<T> head = null;
    size = 0;
    // See merge phase of merge sort for linked lists
    // with the only difference in that this implementations
    // reverts the list during merge
    while (left != null || right != null)
    {
        TreeNode<T> next;
        if (left == null)
            next = DetachAndAdvance(ref right);
        else if (right == null)
            next = DetachAndAdvance(ref left);
        else
            next = comparer.Compare(left.Value, right.Value) > 0
                        ? DetachAndAdvance(ref left)
                        : DetachAndAdvance(ref right);
        next.Right = head;
        head = next;
        size++;
    }
    return head;
}



static TreeNode<T> DetachAndAdvance<T>(ref TreeNode<T> node)
{
    var tmp = node;
    node = node.Left;
    tmp.Left = null;
    return tmp;
}

// Converts singly linked list into binary search tree
// advancing list head to next unused list node and
// returning created tree root
static TreeNode<T> ToBinarySearchTree<T>(ref TreeNode<T> head, int size)
{
    if (size == 0)
        // Zero sized list converts to null
        return null;

    TreeNode<T> root;
    if (size == 1)
    {
        // Unit sized list converts to a node with
        // left and right pointers set to null
        root = head;
        // Advance head to next node in list
        head = head.Right;
        // Left pointers were so only right needs to
        // be nullified
        root.Right = null;
        return root;
    }

    var leftSize = size / 2;
    var rightSize = size - leftSize - 1;
    // Create left substree out of half of list nodes
    var leftRoot = ToBinarySearchTree(ref head, leftSize);
    // List head now points to the root of the subtree
    // being created
    root = head;
    // Advance list head and the rest of the list will
    // be used to create right subtree
    head = head.Right;
    // Link left subtree to the root
    root.Left = leftRoot;
    // Create right subtree and link it to the root
    root.Right = ToBinarySearchTree(ref head, rightSize);
    return root;
}

public static TreeNode<T> Merge<T>(TreeNode<T> left, TreeNode<T> right, IComparer<T> comparer)
{
    Contract.Requires(comparer != null);

    if (left == null || right == null)
        return left ?? right;
    // Convert both trees to sorted lists using original tree nodes
    var leftList = ToSortedList(left, null);
    var rightList = ToSortedList(right, null);
    int size;
    // Merge sorted lists and calculate merged list size
    var list = MergeAsSortedLists(leftList, rightList, comparer, out size);
    // Convert sorted list into optimal binary search tree
    return ToBinarySearchTree(ref list, size);
}

Answer 3

我们可以合并树木的最佳方式是：

For each node n in first BST {
    Go down the 2nd tree and find the appropriate place to insert n
    Insert n there
}

for循环中的每次迭代都是O（log n），因为我们正在处理树，而for循环将被迭代n次，所以我们总共有O（n log n）。

Answer 4

BST是有序或排序的二叉树。我的算法很简单：

遍历两棵树
比较值
将两者中较小的一个插入新的BST。

遍历的 python 代码如下：

def traverse_binary_tree(node, callback):
    if node is None:
        return
    traverse_binary_tree(node.leftChild, callback)
    callback(node.value)
    traverse_binary_tree(node.rightChild, callback)

遍历BST并建立新的合并BST的成本将保持 O（n）

Answer 5

This blog post提供了O（logn）空间复杂度问题的解决方案。（注意给定的方法不会修改输入树。）

Answer 6

这就是我要做的。

此解决方案的时间复杂度为O（n1 + n2）。

STEPS：

对两棵树进行有序遍历以获得排序的数组->线性时间
合并两个数组->再次线性时间
将合并后的数组转换为平衡二进制搜索树->再次线性化时间

这将需要O（n1 + n2）的时间和空间。

在实施时您可能会发现有用的链接

Answer 7

以下算法来自Algorithms in C++。

这个想法与PengOne发布的算法几乎相同。该算法进行了合并，时间复杂度为O（n + m）。

link join(link a, link b) {
    if (b == 0) return a;
    if (a == 0) return b;
    insert(b, a->item);
    b->left = join(a->left, b->left);
    b->right = join(a->right, b->right);
    delete a;
    return b;
}

insert只需在树中的正确位置插入一个项目。

void insert(link &h, Item x) {
    if (h == 0) {
        h = new node(x);
        return;
    }
    if (x.key() < h->item.key()) {
        insert(h->left, x);
        rotateRight(h);
    }
    else {
        insert(h->right, x);
        rotateLeft(h);
    }
}

rotateRight和rotateLeft保持树的顺序正确。

void rotateRight(link &h) {
    link x = h->left;
    h->left = x->right;
    x->right = h;
    h = x;
}

void rotateLeft(link &h) {
    link x = h->right;
    h->right = x->left;
    x->left = h;
    h = x;
}

此处link为node *。

Answer 8

假设问题只是打印从两个BST排序。然后更简单的方法是，

以2个单独的阵列存储inorder遍历2个BST。
现在问题从第2步中的2个排序数组中合并\打印元素减少了。当m> n时，该合并可以在o（m）中进行，当m时，可以在o（n）中进行

复杂性：o（m + n）辅助空间：2个数组的o（m + n）

Answer 9

<强> MergeTwoBST_to_BalancedBST.java

public class MergeTwoBST_to_BalancedBST {

// arr1 and arr2 are the input arrays to be converted into a binary search
// structure and then merged and then balanced.
int[] arr1 = new int[] { 1, 2, 3, 4, 5, 6, 7, 8 };
int[] arr2 = new int[] { 11, 12, 13, 14, 15, 16, 17, 18 };

BSTNode root1;
BSTNode root2;

// vector object to hold the nodes from the merged unbalanced binary search
// tree.
Vector<BSTNode> vNodes = new Vector<BSTNode>();

/**
 * Constructor to initialize the Binary Search Tree root nodes to start
 * processing. This constructor creates two trees from two given sorted
 * array inputs. root1 tree from arr1 and root2 tree from arr2.
 * 
 * Once we are done with creating the tree, we are traversing the tree in
 * inorder format, to verify whether nodes are inserted properly or not. An
 * inorder traversal should give us the nodes in a sorted order.
 */
public MergeTwoBST_to_BalancedBST() {
    // passing 0 as the startIndex and arr1.length-1 as the endIndex.
    root1 = getBSTFromSortedArray(arr1, 0, arr1.length - 1);
    System.out.println("\nPrinting the first binary search tree");
    inorder(root1); // traverse the tree in inorder format to verify whether
                    // nodes are inserted correctly or not.

    // passing 0 as the startIndex and arr2.length-1 as the endIndex.
    root2 = getBSTFromSortedArray(arr2, 0, arr2.length - 1);
    System.out.println("\nPrinting the second binary search tree");
    inorder(root2); // same here - checking whether the nodes are inserted
                    // properly or not.
}

/**
 * Method to traverse the tree in inorder format. Where it traverses the
 * left child first, then root and then right child.
 * 
 * @param node
 */
public void inorder(BSTNode node) {
    if (null != node) {
        inorder(node.getLeft());
        System.out.print(node.getData() + " ");
        inorder(node.getRight());
    }
}

/**
 * Method to traverse the tree in preorder format. Where it traverses the
 * root node first, then left child and then right child.
 * 
 * @param node
 */
public void preorder(BSTNode node) {
    if (null != node) {
        System.out.print(node.getData() + " ");
        preorder(node.getLeft());
        preorder(node.getRight());
    }
}

/**
 * Creating a new Binary Search Tree object from a sorted array and
 * returning the root of the newly created node for further processing.
 * 
 * @param arr
 * @param startIndex
 * @param endIndex
 * @return
 */
public BSTNode getBSTFromSortedArray(int[] arr, int startIndex, int endIndex) {
    if (startIndex > endIndex) {
        return null;
    }

    int middleIndex = startIndex + (endIndex - startIndex) / 2;
    BSTNode node = new BSTNode(arr[middleIndex]);
    node.setLeft(getBSTFromSortedArray(arr, startIndex, middleIndex - 1));
    node.setRight(getBSTFromSortedArray(arr, middleIndex + 1, endIndex));
    return node;
}

/**
 * This method involves two operation. First - it adds the nodes from root1
 * tree to root2 tree, and hence we get a merged root2 tree.Second - it
 * balances the merged root2 tree with the help of a vector object which can
 * contain objects only of BSTNode type.
 */
public void mergeTwoBinarySearchTree() {
    // First operation - merging the trees. root1 with root2 merging should
    // give us a new root2 tree.
    addUtil(root1);
    System.out.println("\nAfter the root1 tree nodes are added to root2");
    System.out.println("Inorder Traversal of root2 nodes");
    inorder(root2); // inorder traversal of the root2 tree should display
                    // the nodes in a sorted order.

    System.out.println("\nPreorder traversal of root2 nodes");
    preorder(root2);

    // Second operation - this will take care of balancing the merged binary
    // search trees.
    balancedTheMergedBST();
}

/**
 * Here we are doing two operations. First operation involves, adding nodes
 * from root2 tree to the vector object. Second operation involves, creating
 * the Balanced binary search tree from the vector objects.
 */
public void balancedTheMergedBST() {
    // First operation : adding nodes to the vector object
    addNodesToVector(root2, vNodes);
    int vSize = vNodes.size();

    // Second operation : getting a balanced binary search tree
    BSTNode node = getBalancedBSTFromVector(vNodes, 0, vSize - 1);
    System.out
            .println("\n********************************************************");
    System.out.println("After balancing the merged trees");
    System.out.println("\nInorder Traversal of nodes");
    inorder(node); // traversing the tree in inoder process should give us
                    // the output in sorted order ascending
    System.out.println("\nPreorder traversal of root2 nodes");
    preorder(node);
}

/**
 * This method will provide us a Balanced Binary Search Tree. Elements of
 * the root2 tree has been added to the vector object. It is parsed
 * recursively to create a balanced tree.
 * 
 * @param vNodes
 * @param startIndex
 * @param endIndex
 * @return
 */
public BSTNode getBalancedBSTFromVector(Vector<BSTNode> vNodes,
        int startIndex, int endIndex) {
    if (startIndex > endIndex) {
        return null;
    }

    int middleIndex = startIndex + (endIndex - startIndex) / 2;
    BSTNode node = vNodes.get(middleIndex);
    node.setLeft(getBalancedBSTFromVector(vNodes, startIndex,
            middleIndex - 1));
    node.setRight(getBalancedBSTFromVector(vNodes, middleIndex + 1,
            endIndex));

    return node;
}

/**
 * This method traverse the tree in inorder process and adds each node from
 * root2 to the vector object vNodes object only accepts objects of BSTNode
 * type.
 * 
 * @param node
 * @param vNodes
 */
public void addNodesToVector(BSTNode node, Vector<BSTNode> vNodes) {
    if (null != node) {
        addNodesToVector(node.getLeft(), vNodes);
        // here we are adding the node in the vector object.
        vNodes.add(node);
        addNodesToVector(node.getRight(), vNodes);
    }
}

/**
 * This method traverse the root1 tree in inorder process and add the nodes
 * in the root2 tree based on their value
 * 
 * @param node
 */
public void addUtil(BSTNode node) {
    if (null != node) {
        addUtil(node.getLeft());
        mergeToSecondTree(root2, node.getData());
        addUtil(node.getRight());
    }
}

/**
 * This method adds the nodes found from root1 tree as part it's inorder
 * traversal and add it to the second tree.
 * 
 * This method follows simple Binary Search Tree inserstion logic to insert
 * a node considering the tree already exists.
 * 
 * @param node
 * @param data
 * @return
 */
public BSTNode mergeToSecondTree(BSTNode node, int data) {

    if (null == node) {
        node = new BSTNode(data);
    } else {
        if (data < node.getData()) {
            node.setLeft(mergeToSecondTree(node.getLeft(), data));
        } else if (data > node.getData()) {
            node.setRight(mergeToSecondTree(node.getRight(), data));
        }
    }

    return node;
}

/**
 * 
 * @param args
 */
public static void main(String[] args) {
    MergeTwoBST_to_BalancedBST mergeTwoBST = new MergeTwoBST_to_BalancedBST();
    mergeTwoBST.mergeTwoBinarySearchTree();
  }
}

<强> BSTNode.java：

public class BSTNode {

BSTNode left, right;
int data;

/* Default constructor */
public BSTNode() {
    left = null;
    right = null;
    data = 0;
}

/* Constructor */
public BSTNode(int data) {
    left = null;
    right = null;
    this.data = data;
}

public BSTNode getLeft() {
    return left;
}

public void setLeft(BSTNode left) {
    this.left = left;
}

public BSTNode getRight() {
    return right;
}

public void setRight(BSTNode right) {
    this.right = right;
}

public int getData() {
    return data;
}

public void setData(int data) {
    this.data = data;
  }
}

Answer 10

这可以通过3个步骤完成：

将BST覆盖到已排序的链表中（可以在O（m + n）时间完成此操作）

将这两个排序的链接列表合并到一个列表中（可以在O（m + n）时间就地完成）

将排序后的链表转换为平衡的BST（这可以在O（m + n）时间完成）

合并2个二叉搜索树

10 个答案: