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- class Solution {
- List<List<Integer>> levels = new ArrayList<List<Integer>>();
- public void helper(TreeNode node, int level) {
- // start the current level
- if (levels.size() == level)
- levels.add(new ArrayList<Integer>());
- // fulfil the current level
- levels.get(level).add(node.val);
- // process child nodes for the next level
- if (node.left != null)
- helper(node.left, level + 1);
- if (node.right != null)
- helper(node.right, level + 1);
- }
- public List<List<Integer>> levelOrder(TreeNode root) {
- if (root == null) return levels;
- helper(root, 0);
- return levels;
- }
- }
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