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],
3
/ \
9 20
/ \
15 7
return its bottom-up level order traversal as:
[
[15,7],
[9,20],
[3]
]
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
public List<List<Integer>> levelOrderBottom(TreeNode root) {
List<List<Integer>> list = new LinkedList<>();
Queue<TreeNode> queue = new LinkedList<>();
if(root==null){
return list;
}
queue.offer(root);
while(queue.size()!=0){
int size = queue.size();
List<Integer> subList = new LinkedList<>();
for(int i=0;i<size;i++){
if(queue.peek().left!=null){
queue.add(queue.peek().left);
}
if(queue.peek().right!=null){
queue.add(queue.peek().right);
}
subList.add(queue.poll().val);
}
list.add(0,subList);
}
return list;
}
}JAVA List在指定位置插入元素：list.add(2, "f");