从下向上层次遍历二叉树，二叉树层次遍历加上层次控制。
107. Binary Tree Level Order Traversal II
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],
return its bottom-up level order traversal as:
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[
[15,7],
[9,20],
[3]
]
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public List<List<Integer>> levelOrderBottom(TreeNode root) {
if (root == null) return new ArrayList<>();
ArrayDeque<TreeNode> queue = new ArrayDeque<>();
List<List<Integer>> ans = new LinkedList<>();
queue.offer(root);
while (!queue.isEmpty()) {
int size = queue.size();
LinkedList<Integer> level = new LinkedList<>();
for (int i = 0; i < size; i++) {
TreeNode pNode = queue.poll();
if (pNode.left != null) queue.offer(pNode.left);
if (pNode.right != null) queue.offer(pNode.right);
level.add(pNode.val);
}
ans.add(0, level);
}
return ans;
}
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