Day-93

vector<int> leftView(Node *root)
{
   // Your code here
   
   if(!root)
    return {};
   
   typedef pair<Node *, int> pi;
   queue<pi> qu;
   vector<int> ans;
   int currLevel = 0, prevLevel = -1;
   
   qu.push({root, 0});   // root is at 0 level
   
   while(!qu.empty()){
        pi temp = qu.front();  
        qu.pop();
        
        currLevel = temp.second;
        
        // if level of two consecutive nodes are different
        // it means the node having higher level is the
        // first in its level and should be inserted in ans.
        
        if(currLevel != prevLevel){
            ans.push_back(temp.first->data);
            prevLevel = currLevel;
        }
        
        // level of children is parent+1
        
        if(temp.first->left)
           qu.push({temp.first->left, temp.second + 1});
        
        if(temp.first->right)
           qu.push({temp.first->right, temp.second + 1});
        
   }
   return ans;
   
}