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;
}
}
//非遞歸
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;
}
}