Inorder Successor in BST.cpp

//{ Driver Code Starts
#include <bits/stdc++.h>
using namespace std;

// Tree Node
struct Node
{
    int data;
    Node *left;
    Node *right;

    Node(int val)
    {
        data = val;
        left = right = NULL;
    }
};
// Function to Build Tree
Node *buildTree(string str)
{
    // Corner Case
    if (str.length() == 0 || str[0] == 'N')
        return NULL;

    // Creating vector of strings from input
    // string after spliting by space
    vector<string> ip;

    istringstream iss(str);
    for (string str; iss >> str;)
        ip.push_back(str);

    // Create the root of the tree
    Node *root = new Node(stoi(ip[0]));

    // Push the root to the queue
    queue<Node *> queue;
    queue.push(root);

    // Starting from the second element
    int i = 1;
    while (!queue.empty() && i < ip.size())
    {

        // Get and remove the front of the queue
        Node *currNode = queue.front();
        queue.pop();

        // Get the current node's value from the string
        string currVal = ip[i];

        // If the left child is not null
        if (currVal != "N")
        {

            // Create the left child for the current node
            currNode->left = new Node(stoi(currVal));

            // Push it to the queue
            queue.push(currNode->left);
        }

        // For the right child
        i++;
        if (i >= ip.size())
            break;
        currVal = ip[i];

        // If the right child is not null
        if (currVal != "N")
        {

            // Create the right child for the current node
            currNode->right = new Node(stoi(currVal));

            // Push it to the queue
            queue.push(currNode->right);
        }
        i++;
    }

    return root;
}

Node *search(Node *root, int key)
{
    // Base Cases: root is null or key is present at root
    if (root == NULL || root->data == key)
        return root;

    // Key is greater than root's key
    if (root->data < key)
        return search(root->right, key);

    // Key is smaller than root's key
    return search(root->left, key);
}

// } Driver Code Ends
/*The structure of Node

struct Node {
    int data;
    Node *left;
    Node *right;

    Node(int val) {
        data = val;
        left = right = NULL;
    }
};
*/

class Solution
{
public:
    // returns the inorder successor of the Node x in BST (rooted at 'root')
    Node *inOrderSuccessor(Node *root, Node *x)
    {
        Node *succ;
        while (root)
        {
            if (x->data >= root->data)
                root = root->right;
            else
            {
                succ = root;
                root = root->left;
            }
        }
        return succ;
    }
};

//{ Driver Code Starts.

int main()
{
    int t;
    cin >> t;
    getchar();

    while (t--)
    {
        string s;
        getline(cin, s);
        Node *head = buildTree(s);

        int k;
        cin >> k;
        getchar();

        Node *kNode = search(head, k);
        Solution obj;
        Node *suc = obj.inOrderSuccessor(head, kNode);

        if (suc == NULL)
            cout << "-1";
        else
            cout << suc->data;
        cout << endl;
    }

    return 1;
}
// } Driver Code Ends