给定一个二叉树,返回其节点值自底向上的层序遍历。 (即按从叶子节点所在层到根节点所在的层,逐层从左向右遍历)
例如:
给定二叉树 [3,9,20,null,null,15,7]
,
3
/ \
9 20
/ \
15 7
返回其自底向上的层序遍历为:
[
[15,7],
[9,20],
[3]
]
\n👍 431👎 0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70
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class Solution { public List<List<Integer>> levelOrderBottom(TreeNode root) { List<List<Integer>> levelOrder = new LinkedList<List<Integer>>(); if (root == null) { return levelOrder; } Queue<TreeNode> queue = new LinkedList<TreeNode>(); queue.offer(root); while (!queue.isEmpty()) { List<Integer> level = new ArrayList<Integer>(); int size = queue.size(); for (int i = 0; i < size; i++) { TreeNode node = queue.poll(); level.add(node.val); TreeNode left = node.left, right = node.right; if (left != null) { queue.offer(left); } if (right != null) { queue.offer(right); } } levelOrder.add(0, level); } return levelOrder; } }
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