Find the left view of a binary tree.

Question: Given the root pointer to a binary tree, find the left view of the tree.

Input: Sample Tree (Pointer to node 1 is given).
Output: 1, 2, 4

At a single glance of the problem, we can understand that we need to perform a level order traversal on the tree. This is similar to finding the number of levels in a tree. At the start of each level, we just print the first element.
This can be done with the algorithm below:-

void leftViewOfBinaryTree(struct binaryTreeNode * root)
{
	// Level order traversal
	struct binaryTreeNode * temp = NULL;
	struct queue * Q = NULL;
	
	// Maintain a level count
	int level = 0;

	if(root == NULL)
		return;

	Q = enQueue(Q, root);
	
	//print the root
	printf("%d ",root->data);
	
	// a flag to check if we need to print
	int needToPrint = 0;
	
	// Now the first level will end over here,
	// So append a NULL node
	Q = enQueue(Q, NULL);
	
	while(!isQueueEmpty(Q))
	{
		temp = Q -> front -> data;
		Q = deQueue(Q);
		
		if(needToPrint)
		{
			printf("%d ", temp->data);
			
			// toggle the flag
			needToPrint = 0;
		}
		
		// If we encounter a NULL, that means an end of a level
		// And we need to increment the level count
		if(temp == NULL)
		{
			// Put the marker for next level also
			if(!isQueueEmpty(Q))
				Q = enQueue(Q, NULL);
			
			level++;
			
			// We are starting a new level, so we will toggle the flag.
			needToPrint = 1;
		}
		else
		{
			// We continue with the level order traversal
			if(temp -> left)
				Q = enQueue(Q, temp -> left);
			if(temp -> right)
				Q = enQueue(Q, temp -> right);
		}
	}
	// Delete the queue
	free(Q);
}

Time Complexity:- O(n)
Space Complexity:- O(n)

Ideone link for the running code:- http://ideone.com/PaJPER