GFG_TopViewBT.cpp

/*
https://practice.geeksforgeeks.org/problems/top-view-of-binary-tree/1
Top View of Binary Tree 
*/

// { Driver Code Starts
//Initial Template for C++

#include <bits/stdc++.h>
using namespace std;

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

// Utility function to create a new Tree Node
Node* newNode(int val)
{
    Node* temp = new Node;
    temp->data = val;
    temp->left = NULL;
    temp->right = NULL;

    return temp;
}

// 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 = newNode(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 = newNode(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 = newNode(stoi(currVal));

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

    return root;
}


 // } Driver Code Ends


/*
struct Node
{
    int data;
    Node* left;
    Node* right;
};
*/
class Solution
{
    public:
    //Function to return a list of nodes visible from the top view 
    //from left to right in Binary Tree.
    vector<int> topView(Node *root)
    {
        // return topViewBFS(root);
        map<int, pair<int, int>> mp;
        topViewDFS(root, 0, 0, mp);
        vector<int> ans;
        for(auto x: mp)
            ans.push_back(x.second.second);
        return ans;
    }
    
    void topViewDFS(Node* root, int dist, int height, map<int, pair<int, int>>& mp)
    {
        if(!root)
            return;
        
        if(mp.find(dist) == mp.end())
            mp[dist] = {height, root->data};
        else //if(mp.find(dist) != mp.end())
        {
            if(height < mp[dist].first)
                mp[dist] = {height, root->data}; //replace the data with less height
        }
        
        // if(root->left)
            topViewDFS(root->left, dist-1, height+1, mp);
        // if(root->right)
            topViewDFS(root->right, dist+1, height+1, mp);
    }
    
    
    vector<int> topViewBFS(Node *root)
    {
        map<int,int> um; // to map the nodes
        //BFS Level Order Traversal
        queue<pair<Node*, int>> q;
        q.push({root, 0}); // vertical distance
        
        int sz, dist;
        Node * node;
        while(!q.empty())
        {
             sz = q.size();
            while(sz--)
            {
                node = q.front().first;
                dist = q.front().second; q.pop();
                if(node->left)
                    q.push({node->left, dist-1});
                if(node->right)
                    q.push({node->right, dist+1});
                
                if(um.find(dist) == um.end())
                    um[dist] = node->data;
            }
        }
        vector<int> ans;
        for(auto x: um)
            ans.push_back(x.second);
        return ans;
    }
    
    
};



// { Driver Code Starts.

int main() {
    int tc;
    cin>>tc;
    cin.ignore(256, '\n');
    while (tc--) {
        string treeString;
        getline(cin, treeString);
        Solution ob;
        Node *root = buildTree(treeString);
        vector<int> vec = ob.topView(root);
        for(int x : vec)
            cout<<x<<" ";
        cout<<endl;
    }
    return 0;
}  // } Driver Code Ends