BFS template - 199

199.java

class Solution {
    // Tree level-traversal
    // Graph - BFS
    public List<Integer> rightSideView(TreeNode root) {
        if (root == null) return Collections.emptyList();
        
        List<Integer> results = new ArrayList<>();
        
        Deque<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        
        while (!queue.isEmpty()) {
            int size = queue.size();
            // System.out.println(queue);
            
            // start looping elements of the current level
            // this means the queue has all elements for the current level
            results.add(queue.getLast().val);
            
            for (int i = 0; i < size; i++) {
                TreeNode node = queue.poll();    
                if (node.left != null) queue.add(node.left);
                if (node.right != null) queue.add(node.right);
            }
        }
        
        return results;
    }
}