描述
给出一棵二叉树，返回其节点值从底向上的层次序遍历（按从叶节点所在层到根节点所在的层遍历，然后逐层从左往右遍历）
样例
给出一棵二叉树 {3,9,20,#,#,15,7},
3
/ \
9 20
/ \
15 7
按照从下往上的层次遍历为：
[
[15,7],
[9,20],
[3]
]
代码
/**
* Definition of TreeNode:
* public class TreeNode {
* public int val;
* public TreeNode left, right;
* public TreeNode(int val) {
* this.val = val;
* this.left = this.right = null;
* }
* }
*/
public class Solution {
/**
* @param root: The root of binary tree.
* @return: buttom-up level order a list of lists of integer
*/
public List<List<Integer>> levelOrderBottom(TreeNode root) {
List<List<Integer>> result = new ArrayList<>();
if (root == null) {
return result;
}
Queue<TreeNode> queue = new LinkedList<TreeNode>();
queue.offer(root);
while (!queue.isEmpty()) {
int size = queue.size();
List<Integer> level = new ArrayList<>();
for (int i = 0; i < size; i++) {
TreeNode head = queue.poll();
level.add(head.val);
if (head.left != null) {
queue.offer(head.left);
}
if (head.right != null) {
queue.offer(head.right);
}
}
result.add(level);
}
// 方法被用来反转指定列表中的元素的顺序，时间复杂度O(n)
Collections.reverse(result);
return result;
}
}
