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,#,#,15,7},

    3
   / \
  9  20
    /  \
   15   7

 

return its bottom-up level order traversal as:

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

 

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.

---

同basic travel by level, 只不过最后把arraylist reverse一下

---

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ArrayList<ArrayList<Integer>> levelOrderBottom(TreeNode root) {
        ArrayList<ArrayList<Integer>> rst = new ArrayList<ArrayList<Integer>>();     
        helper(root, 0, rst);
        //reverse the arraylist
        Collections.reverse(rst);
        return rst;
        
    }
    
    
    private void helper(TreeNode node,  int level, 
            ArrayList<ArrayList<Integer>> rst){
        
        if(node == null) return;
        
        ArrayList<Integer> list = null;
        
        if(rst.size() == level){ // a new level
            list = new ArrayList<Integer>();
            list.add(node.val);
            rst.add(list);
            
        }else{
            
            list = rst.get(level);
            list.add(node.val);
        }
        
        helper(node.left, level+1, rst);
        helper(node.right, level+1, rst);
    }
}