[LeetCode] Binary Tree Level Order Traversal II

3
   / \
  9  20
    /  \
   15   7

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

/**
 * 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) {
        //require
        List<List<Integer>> ans=new LinkedList<>();
        Queue<TreeNode> q=new LinkedList<>();
        if(root!=null)q.add(root);
        //invariant
        while(!q.isEmpty()){
           int size=q.size();
           List<Integer> list=new LinkedList<>();
           for(int i=0;i<size;i++){
               TreeNode n=q.remove();
               list.add(n.val);
               if(n.left!=null)q.add(n.left);
               if(n.right!=null)q.add(n.right);
           }
           ans.add(0,list);
        }
        //ensure
        return ans;
    }
}