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/* A binary tree node

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

//Function to return a list containing elements of left view of the binary tree.
vector<int> leftView(Node *root)
{
    vector<int> leftView;
    
    if (root == NULL) return leftView;
    
    queue<Node *> q;
    q.push(root);
    
    while (!q.empty()) {
        int levelSize = q.size();
        
        for (int i = 1; i <= levelSize; i++) {
            Node *curr = q.front();
            q.pop();
            
            if (i == 1)
                leftView.push_back(curr -> data);
                
            if (curr -> left != NULL)
                q.push(curr -> left);
            if (curr -> right != NULL)
                q.push(curr -> right);
        }
    }
    
    return leftView;
}