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).
Example：
Given binary tree [3,9,20,null,null,15,7]
[
[15,7],
[9,20],
[3]
]
解释下题目：
就是把一棵二叉树用list表示
1. 使用队列
实际耗时：2ms
public class Solution {
public List<List<Integer>> levelOrderBottom(TreeNode root) {
List<List<Integer>> res = new ArrayList<>();
if (root == null) {
return res;
}
Queue<TreeNode> q = new LinkedList<>();
q.add(root);
while (q.size() > 0) {
List<Integer> list = new ArrayList<>();
int size = q.size();
for (int i = 0; i < size; i++) {
TreeNode node = q.poll();
list.add(node.val);
if (node.left != null) q.add(node.left);
if (node.right != null) q.add(node.right);
}
res.add(0, list);
}
return res;
}
}
思路就是用队列按序做
