Basic Calculator LeetCode solution in Java

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class Solution {
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  int idx; // this index traverse the string in one pass, between different level of recursion
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  public int calculate(String s) {
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    idx = 0; // Initialization should be here
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    return helper(s);
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  }
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  private int helper(String s) {
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    int res = 0, num = 0, preSign = 1, n = s.length();
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    while (idx < n) {
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      char c = s.charAt(idx++);
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      if (c == '(') num = helper(s); // Let recursion solve the sub-problem
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      else if (c >= '0' && c <= '9') num = num * 10 + c - '0';
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      if (c == '+' || c == '-' || c == ')'
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          || idx == n) { // we need one more calculation when idx == n
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        res += preSign * num;
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        if (c == ')') return res; // end of a sub-problem
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        num = 0;
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        preSign = c == '+' ? 1 : -1;
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      }
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    }
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    return res;
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  }
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}