Decoded String at Index LeetCode solution in Java

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class Solution {
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  public String decodeAtIndex(String s, int k) {
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    long sz = 0;
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    for (char ch : s.toCharArray()) { // total length
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      sz = Character.isDigit(ch) ? sz * (ch - '0') : ++sz;
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    }
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    --k; // make it 0 index based.
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    for (int i = s.length() - 1; true; i--) {
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      if (Character.isLetter(s.charAt(i)) && --sz == k) { // found!
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        return "" + s.charAt(i);
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      } else if (Character.isDigit(s.charAt(i))) {
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        sz /= (s.charAt(i) - '0');
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        k %= sz; // we are at the end of a multplied string, we can mod k with sz.
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      }
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    }
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  }
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}