description: Given a binary tree, return the bottom-up level order traversal of its nodes’ values. (ie, from left to right, level by level from leaf to root).
For example: Given binary tree [3,9,20,null,null,15,7],
3 / / 9 20 / /15 7return its bottom-up level order traversal as: [ [15,7], [9,20], [3] ] 题目并不复杂，只是在binary tree level order traversal 的基础之上进行的一个follow up
/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */public class Solution { public List<List<Integer>> levelOrderBottom(TreeNode root) { List<List<Integer>> result = new ArrayList<>(); if (root == null) { return result; } Queue<TreeNode> queue = new LinkedList<>(); queue.add(root); while(!queue.isEmpty()) { List<Integer> level = new ArrayList<>(); int len = queue.size(); for (int i = 0; i < len; i++) { TreeNode node = queue.poll(); level.add(node.val); if (node.left != null) { queue.offer(node.left); } if (node.right != null) { queue.offer(node.right); } } result.add(level); } List<List<Integer>> results = new ArrayList<>(); for(int i = result.size() - 1; i >= 0; i--) { results.add(result.get(i)); } return results; }}新闻热点
疑难解答
