Number of Nodes in the Sub-Tree With the Same Label LeetCode solution in Java

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class Solution {
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  int[] res;
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  public int[] countSubTrees(int n, int[][] edges, String labels) {
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    res = new int[n];
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    Map<Integer, List<Integer>> adjList = new HashMap<>();
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    for (int i = 0; i < n; i++) {
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      adjList.put(i, new ArrayList<>());
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    }
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    for (int[] edge : edges) {
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      adjList.get(edge[0]).add(edge[1]);
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      adjList.get(edge[1]).add(edge[0]);
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    }
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    postOrderDfs(adjList, labels, 0, -1);
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    return res;
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  }
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  int[] postOrderDfs(Map<Integer, List<Integer>> adjList, String labels, int n, int parent) {
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    int[] chars = new int[26];
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    chars[labels.charAt(n) - 'a']++;
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    for (int next : adjList.get(n)) {
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      if (next != parent) mergeArrCounts(chars, postOrderDfs(adjList, labels, next, n));
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    }
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    res[n] = chars[labels.charAt(n) - 'a'];
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    return chars;
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  }
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  // Merge from B to A
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  void mergeArrCounts(int[] A, int[] B) {
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    for (int i = 0; i < 26; i++) {
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      A[i] += B[i];
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    }
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  }
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}