107.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        List<List<Integer>> result = new LinkedList<List<Integer>>();
        if(root==null) return result;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while(queue.size()>0){
            List<Integer> temp = new ArrayList<Integer>();
            int len = queue.size();
            for(int i=0;i<len;i++){
                TreeNode node = queue.poll();
                temp.add(node.val);
                if(node.left!=null) queue.offer(node.left);
                if(node.right!=null) queue.offer(node.right);
            }
            result.add(0,temp);
        }
        return result;
    }
}