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]
]
与之前的题做法一样，就是插入的时候，每次在第一位插入。
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public List<List<Integer>> levelOrderBottom(TreeNode root) { List list = new ArrayList<List<Integer>>(); if( root == null)
return list; Queue<TreeNode> tree = new LinkedList<TreeNode>(); tree.add(root);
while( !tree.isEmpty() ){
List ans = new ArrayList<Integer>();
int size = tree.size();
for( int i = 0;i<size;i++){
TreeNode node = tree.poll();
ans.add(node.val);
if( node.left != null )
tree.add(node.left);
if( node.right != null)
tree.add(node.right);
}
if( !ans.isEmpty() )
list.add(0,ans); } return list; }
}