Binary Tree Level Order Traversal II 解答

Question

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,#,#,15,7},

return its bottom-up level order traversal as:

[
  [15,7],
  [9,20],
  [3]
]

Solution

Same method as "Binary Tree Level Order Traversal". Note that for ArrayList, it has function add(int index, E element).

 1 /**
 2  * Definition for a binary tree node.
 3  * public class TreeNode {
 4  *     int val;
 5  *     TreeNode left;
 6  *     TreeNode right;
 7  *     TreeNode(int x) { val = x; }
 8  * }
 9  */
10 public class Solution {
11     public List<List<Integer>> levelOrderBottom(TreeNode root) {
12         List<List<Integer>> result = new ArrayList<List<Integer>>();
13         if (root == null)
14             return result;
15         Deque<TreeNode> prev = new ArrayDeque<TreeNode>();
16         Deque<TreeNode> current;
17         TreeNode tmpNode;
18         prev.addLast(root);
19         while (prev.size() > 0) {
20             current = new ArrayDeque<TreeNode>();
21             List<Integer> tmpList = new ArrayList<Integer>();
22             while (prev.size() > 0) {
23                 tmpNode = prev.pop();
24                 if (tmpNode.left != null)
25                     current.addLast(tmpNode.left);
26                 if (tmpNode.right != null)
27                     current.addLast(tmpNode.right);
28                 tmpList.add(tmpNode.val);
29             }
30             prev = current;
31             result.add(0, tmpList);
32         }
33         return result;        
34     }
35 }