Binary Tree Postorder Traversal 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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  List<Integer> res = new ArrayList<>();
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  public List<Integer> postorderTraversal(TreeNode root) {
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    traversal(root);
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    return res;
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  }
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  public void traversal(TreeNode root) {
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    if (root == null) return;
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    traversal(root.left);
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    traversal(root.right);
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    res.add(root.val);
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  }
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}