Find Kth Bit in Nth Binary String LeetCode solution in Java

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class Solution {
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  private String invert(String s) {
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    char[] array = s.toCharArray();
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    for (int i = 0; i < s.length(); i++) {
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      if (array[i] == '1') {
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        array[i] = '0';
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      } else {
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        array[i] = '1';
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      }
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    }
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    return new String(array);
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  }
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  private String reverse(String s) {
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    StringBuilder str = new StringBuilder(s);
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    return str.reverse().toString();
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  }
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  private String func(int i) {
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    if (i == 0) {
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      return "0";
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    }
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    return func(i - 1) + "1" + reverse(invert(func(i - 1)));
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  }
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  public char findKthBit(int n, int k) {
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    String s = func(n - 1);
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    return s.charAt(k - 1);
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  }
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}