解法1 - 递归
class Solution {
List<List<Integer>> levels = new ArrayList<List<Integer>>();
public List<List<Integer>> levelOrder(TreeNode root) {
if (root == null) return levels;
helper(root, 0);
return levels;
}
public void helper(TreeNode node, int level) {
if (levels.size() == level)
levels.add(new ArrayList<Integer>());
levels.get(level).add(node.val);
if (node.left != null) helper(node.left, level + 1);
if (node.right != null) helper(node.right, level + 1);
}
}
解法2 - 非递归
class Solution {
public List<List<Integer>> levelOrder(TreeNode root) {
List<List<Integer>> levels = new ArrayList<>();
if (root == null) return levels;
Queue<TreeNode> queue = new LinkedList<TreeNode>();
queue.offer(root);
int level = 0;
while (!queue.isEmpty()) {
levels.add(new ArrayList<Integer>());
int size = queue.size();
while (size > 0) {
TreeNode node = queue.poll();
levels.get(level).add(node.val);
if (node.left != null) queue.offer(node.left);
if (node.right != null) queue.offer(node.right);
size--;
}
level++;
}
return levels;
}
}