107. Binary Tree Level Order Traversal II

Posted 2020-08-09 Machelsky

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;
    }
    
}