Lexicographical Numbers LeetCode solution in Java

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class Solution {
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  private final TrieNode trie = new TrieNode(' ');
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  class TrieNode {
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    private Character digit;
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    private String value;
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    private boolean isWord;
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    private Map<Character, TrieNode> children;
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    TrieNode(Character c) {
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      this.digit = c;
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      this.isWord = false;
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      this.children = new HashMap<>();
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    }
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    void insert(String s) {
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      TrieNode current = this;
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      for (Character c : s.toCharArray()) {
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        current = current.children.computeIfAbsent(c, k -> new TrieNode(c));
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      }
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      current.value = s;
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      current.isWord = true;
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    }
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    List<Integer> getWordsPreOrder() {
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      return getWordsPreOrder(this);
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    }
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    private List<Integer> getWordsPreOrder(TrieNode root) {
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      List<Integer> result = new ArrayList<>();
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      if (root == null) {
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        return result;
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      }
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      if (root.isWord) {
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        result.add(Integer.parseInt(root.value));
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      }
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      for (TrieNode node : root.children.values()) {
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        result.addAll(getWordsPreOrder(node));
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      }
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      return result;
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    }
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  }
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  public List<Integer> lexicalOrder(int n) {
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    for (int i = 1; i <= n; i++) {
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      trie.insert(String.valueOf(i));
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    }
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    return trie.getWordsPreOrder();
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  }
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}