102 Binary Tree Level Order Traversal class Solution { public List<List<Integer>> levelOrder(TreeNode root) { List<List<Integer>> result = new ArrayList<>(); if(root == null) return result; Queue<TreeNode> queue = new LinkedList<>(); // queue, stack interface?? queue.offer(root); while(!queue.isEmpty()){ int size = queue.size(); List<Integer> list = new ArrayList<>(); // create a new list here before the size for(int i = 0; i < size; i++){ TreeNode cur = queue.poll(); list.add(cur.val); if(cur.left != null) queue.offer(cur.left); if(cur.right != null) queue.offer(cur.right); } result.add(list); } return result; } } // stack , push , pop // queue. Offer, poll
102 Binary Tree Level Order Traversal
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