LeetCode第107题 - 二叉树的层序遍历 II

题目

解答

class Solution {List<List<Integer>> nodeLevels = new LinkedList<>();public List<List<Integer>> levelOrderBottom(TreeNode root) {levelOrder(root, 0);List<List<Integer>> nodeLevels2 = new LinkedList<>();for (int i = 0; i < nodeLevels.size(); ++i) {nodeLevels2.add(0, nodeLevels.get(i));}return nodeLevels2;}public void levelOrder(TreeNode root, int k) {if (root == null) {return;}if (nodeLevels.size() <= k) {nodeLevels.add(new ArrayList<>());}nodeLevels.get(k).add(root.val);levelOrder(root.left, k + 1);levelOrder(root.right, k + 1);}
}

要点
层序遍历树时，保留节点的层序的信息。