描述
給出一棵二叉樹，返回其節(jié)點值從底向上的層次序遍歷（按從葉節(jié)點所在層到根節(jié)點所在的層遍歷浸策，然后逐層從左往右遍歷）
樣例
給出一棵二叉樹 {3,9,20,#,#,15,7},
3
/ \
9 20
/ \
15 7
按照從下往上的層次遍歷為：
[
[15,7],
[9,20],
[3]
]
代碼
/**
* 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);
}
// 方法被用來反轉(zhuǎn)指定列表中的元素的順序幔摸，時間復(fù)雜度O(n)
Collections.reverse(result);
return result;
}
}
