Number of Atoms LeetCode solution in Java

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class Solution {
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  public String countOfAtoms(String formula) {
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    Deque<Integer> multiplier = new ArrayDeque<>();
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    Map<String, Integer> map = new HashMap<>();
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    int lastValue = 1;
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    multiplier.push(1);
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    // Iterate from right to left
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    for (int i = formula.length() - 1; i >= 0; i--) {
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      if (Character.isDigit(formula.charAt(i))) { // Case of a digit - Get full number and save
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        StringBuilder sb = new StringBuilder();
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        while (Character.isDigit(formula.charAt(i - 1))) {
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          sb.append(formula.charAt(i));
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          i--;
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        }
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        sb.append(formula.charAt(i));
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        lastValue = Integer.parseInt(sb.reverse().toString());
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      } else if (formula.charAt(i) == ')') { // Start parenthesis - push next multiplier to stack
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        multiplier.push(lastValue * multiplier.peek());
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        lastValue = 1;
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      } else if (formula.charAt(i) == '(') { // End parenthesis - pop last multiplier
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        multiplier.pop();
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      } else { // Case of an element name - construct name, update count based on multiplier
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        StringBuilder sb = new StringBuilder();
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        while (Character.isLowerCase(formula.charAt(i))) {
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          sb.append(formula.charAt(i));
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          i--;
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        }
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        sb.append(formula.charAt(i));
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        String element = sb.reverse().toString();
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        map.put(element, map.getOrDefault(element, 0) + lastValue * multiplier.peek());
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        lastValue = 1;
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      }
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    }
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    // Sort map keys
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    List<String> elements = new ArrayList<>(map.keySet());
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    StringBuilder sb = new StringBuilder();
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    Collections.sort(elements);
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    // Construct output
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    for (String element : elements) {
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      sb.append(element);
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      if (map.get(element) > 1) {
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        sb.append(map.get(element));
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      }
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    }
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    return sb.toString();
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  }
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}