Custom Sort String LeetCode solution in Java

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class Solution {
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  public String customSortString(String X, String Y) {
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    // char count of string Y
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    int[] charCount = new int[26];
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    for (char c : Y.toCharArray()) {
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      charCount[c - 'a']++;
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    }
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    StringBuilder sb = new StringBuilder();
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    // first store char in same order of String X
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    for (char c : X.toCharArray()) {
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      while (charCount[c - 'a']-- > 0) {
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        sb.append(c);
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      }
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    }
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    // now store remaining char of string Y
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    for (int i = 0; i < 26; i++) {
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      char c = (char) ('a' + i);
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      while (charCount[i]-- > 0) {
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        sb.append(c);
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      }
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    }
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    return sb.toString();
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  }
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}