Merge two BSTs with limited extra space

4.3

Given two Binary Search Trees(BST), print the elements of both BSTs in sorted form. The expected time complexity is O(m+n) where m is the number of nodes in first tree and n is the number of nodes in second tree. Maximum allowed auxiliary space is O(height of the first tree + height of the second tree).

Examples:

First BST 
       3
    /     \
   1       5
Second BST
    4
  /   \
2       6
Output: 1 2 3 4 5 6


First BST 
          8
         / \
        2   10
       /
      1
Second BST 
          5
         / 
        3  
       /
      0
Output: 0 1 2 3 5 8 10 

Source: Google interview question

A similar question has been discussed earlier. Let us first discuss already discussed methods of the previous post which was for Balanced BSTs. The method 1 can be applied here also, but the time complexity will be O(n^2) in worst case. The method 2 can also be applied here, but the extra space required will be O(n) which violates the constraint given in this question. Method 3 can be applied here but the step 3 of method 3 can’t be done in O(n) for an unbalanced BST.

Thanks to Kumar for suggesting the following solution.

The idea is to use iterative inorder traversal. We use two auxiliary stacks for two BSTs. Since we need to print the elements in sorted form, whenever we get a smaller element from any of the trees, we print it. If the element is greater, then we push it back to stack for the next iteration.

#include<stdio.h>
#include<stdlib.h>

// Structure of a BST Node
struct node
{
    int data;
    struct node *left;
    struct node *right;
};

//.................... START OF STACK RELATED STUFF....................
// A stack node
struct snode
{
    struct node  *t;
    struct snode *next;
};

// Function to add an elemt k to stack
void push(struct snode **s, struct node *k)
{
    struct snode *tmp = (struct snode *) malloc(sizeof(struct snode));

    //perform memory check here
    tmp->t = k;
    tmp->next = *s;
    (*s) = tmp;
}

// Function to pop an element t from stack
struct node *pop(struct snode **s)
{
    struct  node *t;
    struct snode *st;
    st=*s;
    (*s) = (*s)->next;
    t = st->t;
    free(st);
    return t;
}

// Fucntion to check whether the stack is empty or not
int isEmpty(struct snode *s)
{
    if (s == NULL )
        return 1;

    return 0;
}
//.................... END OF STACK RELATED STUFF....................


/* Utility function to create a new Binary Tree node */
struct node* newNode (int data)
{
    struct node *temp = new struct node;
    temp->data = data;
    temp->left = NULL;
    temp->right = NULL;
    return temp;
}

/* A utility function to print Inoder traversal of a Binary Tree */
void inorder(struct node *root)
{
    if (root != NULL)
    {
        inorder(root->left);
        printf("%d ", root->data);
        inorder(root->right);
    }
}

// The function to print data of two BSTs in sorted order
void  merge(struct node *root1, struct node *root2)
{
    // s1 is stack to hold nodes of first BST
    struct snode *s1 = NULL;

    // Current node of first BST
    struct node  *current1 = root1;

    // s2 is stack to hold nodes of second BST
    struct snode *s2 = NULL;

    // Current node of second BST
    struct node  *current2 = root2;

    // If first BST is empty, then output is inorder
    // traversal of second BST
    if (root1 == NULL)
    {
        inorder(root2);
        return;
    }
    // If second BST is empty, then output is inorder
    // traversal of first BST
    if (root2 == NULL)
    {
        inorder(root1);
        return ;
    }

    // Run the loop while there are nodes not yet printed.
    // The nodes may be in stack(explored, but not printed)
    // or may be not yet explored
    while (current1 != NULL || !isEmpty(s1) ||
          current2 != NULL || !isEmpty(s2))
    {
        // Following steps follow iterative Inorder Traversal
        if (current1 != NULL || current2 != NULL )
        {
            // Reach the leftmost node of both BSTs and push ancestors of
            // leftmost nodes to stack s1 and s2 respectively
            if (current1 != NULL)
            {
                push(&s1, current1);
                current1 = current1->left;
            }
            if (current2 != NULL)
            {
                push(&s2, current2);
                current2 = current2->left;
            }

        }
        else
        {
            // If we reach a NULL node and either of the stacks is empty,
            // then one tree is exhausted, ptint the other tree
            if (isEmpty(s1))
            {
                while (!isEmpty(s2))
                {
                    current2 = pop (&s2);
                    current2->left = NULL;
                    inorder(current2);
                }
                return ;
            }
            if (isEmpty(s2))
            {
                while (!isEmpty(s1))
                {
                    current1 = pop (&s1);
                    current1->left = NULL;
                    inorder(current1);
                }
                return ;
            }

            // Pop an element from both stacks and compare the
            // popped elements
            current1 = pop(&s1);
            current2 = pop(&s2);

            // If element of first tree is smaller, then print it
            // and push the right subtree. If the element is larger,
            // then we push it back to the corresponding stack.
            if (current1->data < current2->data)
            {
                printf("%d ", current1->data);
                current1 = current1->right;
                push(&s2, current2);
                current2 = NULL;
            }
            else
            {
                printf("%d ", current2->data);
                current2 = current2->right;
                push(&s1, current1);
                current1 = NULL;
            }
        }
    }
}

/* Driver program to test above functions */
int main()
{
    struct node  *root1 = NULL, *root2 = NULL;

    /* Let us create the following tree as first tree
            3
          /  \
         1    5
     */
    root1 = newNode(3);
    root1->left = newNode(1);
    root1->right = newNode(5);

    /* Let us create the following tree as second tree
            4
          /  \
         2    6
     */
    root2 = newNode(4);
    root2->left = newNode(2);
    root2->right = newNode(6);

    // Print sorted nodes of both trees
    merge(root1, root2);

    return 0;
}

Time Complexity: O(m+n)
Auxiliary Space: O(height of the first tree + height of the second tree)

Asked in: Amazon, Google, Microsoft

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