Additive Number LeetCode solution in Java

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class Solution {
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  public boolean isAdditiveNumber(String num) {
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    return backtrack(num, 0, 0, 0, 0);
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  }
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  public boolean backtrack(String num, int idx, long sum, long prev, int length) {
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    if (idx == num.length()) {
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      return length >= 3;
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    }
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    long currLong = 0;
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    for (int i = idx; i < num.length(); i++) {
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      // make sure it won't start with 0
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      if (i > idx && num.charAt(idx) == '0') break;
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      currLong = currLong * 10 + num.charAt(i) - '0';
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      if (length >= 2) {
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        if (sum < currLong) {
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          // currLong is greater than sum of previous 2 numbers
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          break;
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        } else if (sum > currLong) {
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          // currLong is smaller than sum of previous 2 numbers
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          continue;
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        }
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      }
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      // currLong == sum of previous 2 numbers
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      if (backtrack(num, i + 1, currLong + prev, currLong, length + 1) == true) {
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        return true;
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      }
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    }
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    return false;
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  }
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}