Binary Tree Level Order Traversal II LeetCode solution in Java

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/**
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 * Definition for a binary tree node. public class TreeNode { int val; TreeNode left; TreeNode
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 * right; TreeNode() {} TreeNode(int val) { this.val = val; } TreeNode(int val, TreeNode left,
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 * TreeNode right) { this.val = val; this.left = left; this.right = right; } }
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 */
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class Solution {
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  public List<List<Integer>> levelOrderBottom(TreeNode root) {
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    Queue<TreeNode> al = new LinkedList<>();
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    List<List<Integer>> fal = new LinkedList<>();
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    if (root == null) return fal;
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    al.offer(root);
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    while (!al.isEmpty()) {
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      List<Integer> aal = new LinkedList<>();
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      int num = al.size();
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      for (int i = 0; i < num; i++) {
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        if (al.peek().left != null) {
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          al.offer(al.peek().left);
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        }
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        if (al.peek().right != null) {
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          al.offer(al.peek().right);
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        }
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        aal.add(al.poll().val);
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      }
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      fal.add(0, aal);
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    }
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    return fal;
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  }
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}