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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]]
思路：跟I一样，把结果插到arraylist但开头就可以。
/** * 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<Integer>> res= new ArrayList<List<Integer>>(); if(root==null) { return res; } Queue<TreeNode> check=new LinkedList<TreeNode>(); check.offer(root); while(!check.isEmpty()) { List<Integer> curl=new ArrayList<Integer>(); for(int i=check.size()-1;i>=0;i--) { TreeNode find=check.poll(); curl.add(find.val); if(find.left!=null) { check.offer(find.left); } if(find.right!=null) { check.offer(find.right); } } res.add(0,curl); } return res; } }
107. Binary Tree Level Order Traversal II
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