Binary Tree Level Order Traversal II

Fair Binary
Tree Level Order Traversal II
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47%

Accepted

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).
Example

Given binary tree 

{3,9,20,#,#,15,7}

,

3
/ \
9  20
/  \
15   7

 

return its bottom-up level order traversal as:

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

1. regular level-order tree traversal, push arraylist of each level into a stack

    pop from the stack to the final arrayList

/**
* 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 ArrayList<ArrayList<Integer>> levelOrderButtom(TreeNode root) {
ArrayList<ArrayList<Integer>> rst = new ArrayList<ArrayList<Integer>>();
if(root == null) {
return rst;
}
LinkedList<ArrayList<Integer>> rstr = new LinkedList<ArrayList<Integer>>();
ArrayList<Integer> level = new ArrayList();
Queue<TreeNode> q = new LinkedList();
q.add(root);
while(!q.isEmpty()){
int size = q.size();
for(int i = 0; i < size; i++) {
TreeNode temp = q.poll();
level.add(temp.val);
if(temp.left != null) {
q.add(temp.left);
}
if(temp.right != null) {
q.add(temp.right);
}
}
rstr.push(new ArrayList(level));
level = new ArrayList<Integer>();
}
while(!rstr.isEmpty()) {
rst.add(rstr.pop());
}
return rst;
}
}

2. when add level to rst, add at the first index everytime. 

/**
* 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 ArrayList<ArrayList<Integer>> levelOrderButtom(TreeNode root) {
ArrayList<ArrayList<Integer>> rst = new ArrayList<ArrayList<Integer>>();
if(root == null) {
return rst;
}
ArrayList<Integer> level = new ArrayList();
Queue<TreeNode> q = new LinkedList();
q.add(root);
while(!q.isEmpty()){
int size = q.size();
for(int i = 0; i < size; i++) {
TreeNode temp = q.poll();
level.add(temp.val);
if(temp.left != null) {
q.add(temp.left);
}
if(temp.right != null) {
q.add(temp.right);
}
}
rst.add(0, new ArrayList(level));
level = new ArrayList<Integer>();
}
return rst;
}
}