Top View of Binary Tree

Given below is a binary tree. The task is to print the top view of binary tree. Top view of a binary tree is the set of nodes visible when the tree is viewed from the top. For the given below tree

• Time : O(n)
• Space : O(n)

class Solution
{
    int mi = 0;
    unordered_map<int, pair<int,int>> mp;
    void helper(Node* node, int w, int dep = 0) {
        if(!node) return;
        mi = min(mi, w);
        if(!mp.count(w) or mp[w].first > dep) mp[w] = {dep, node->data};
        
        helper(node->left, w - 1, dep + 1);
        helper(node->right, w + 1, dep + 1);
    }
    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)
    {
        helper(root, 0);
        vector<int> res;
        while(mp.count(mi)) {
            res.push_back(mp[mi++].second);
        }
        return res;
    }

};