Check if given sorted sub-sequence exists in binary search tree

Given a binary search tree and a sorted sub-sequence. the task is to check if the given sorted sub-sequence exist in binary search tree or not.

200px-Binary_search_tree.svg

Examples:

// For above binary search tree
Input : seq[] = {4, 6, 8, 14}
Output: "Yes"

Input : seq[] = {1, 2, 3, 8}
Output: "No"

A simple solution is to store inorder traversal in an auxiliary array and then by matching elements of sorted sub-sequence one by one with inorder traversal of tree , we can if sub-sequence exist in BST or not. Time complexity for this approach will be O(n) but it requires extra space O(n) for storing traversal in an array.

An efficient solution is to match elements of sub-sequence while we are traversing BST in inorder fashion. We take index as a iterator for given sorted sub-sequence and start inorder traversal of given bst, if current node matches with seq[index] then move index in forward direction by incrementing 1 and after complete traversal of BST if index==n that means all elements of given sub-sequence have been matched and exist as a sorted sub-sequence in given BST.

// C++ program to find if given array exists as a
// subsequece in BST
#include<bits/stdc++.h>
using namespace std;

// A binary Tree node
struct Node
{
    int data;
    struct Node *left, *right;
};

// A utility function to create a new BST node
// with key as given num
struct Node* newNode(int num)
{
    struct Node* temp = new Node;
    temp->data = num;
    temp->left = temp->right = NULL;
    return temp;
}

// A utility function to insert a given key to BST
struct Node* insert(struct Node* root, int key)
{
    if (root == NULL)
        return newNode(key);
    if (root->data > key)
        root->left = insert(root->left, key);
    else
        root->right = insert(root->right, key);
    return root;
}

// function to check if given sorted sub-sequence exist in BST
// index --> iterator for given sorted sub-sequence
// seq[] --> given sorted sub-sequence
void seqExistUtil(struct Node *ptr, int seq[], int &index)
{
    if (ptr == NULL)
        return;

    // We traverse left subtree first in Inorder
    seqExistUtil(ptr->left, seq, index);

    // If current node matches with se[index] then move
    // forward in sub-sequence
    if (ptr->data == seq[index])
        index++;

    // We traverse left subtree in the end in Inorder
    seqExistUtil(ptr->right, seq, index);
}

// A wrapper over seqExistUtil. It returns true
// if seq[0..n-1] exists in tree.
bool seqExist(struct Node *root, int seq[], int n)
{
    // Initialize index in seq[]
    int index = 0;

    // Do an inorder traversal and find if all
    // elements of seq[] were present
    seqExistUtil(root, seq, index);

    // index would become n if all elements of
    // seq[] were present
    return (index == n);
}

// driver program to run the case
int main()
{
    struct Node* root = NULL;
    root = insert(root, 8);
    root = insert(root, 10);
    root = insert(root, 3);
    root = insert(root, 6);
    root = insert(root, 1);
    root = insert(root, 4);
    root = insert(root, 7);
    root = insert(root, 14);
    root = insert(root, 13);

    int seq[] = {4, 6, 8, 14};
    int n = sizeof(seq)/sizeof(seq[0]);

    seqExist(root, seq, n)? cout << "Yes" :
                            cout << "No";

    return 0;
}

Output:

Yes

Time complexity : O(n)

This article is contributed by Shashank Mishra ( Gullu ). If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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