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public class Solution {
    public boolean isAdditiveNumber(String num) {
        if (num == null || num.length() < 3) return false;
        int len = num.length();
        for (int i = 1; i <= len / 2; i++) {
            for (int j = 1; j <= (len - i) / 2; j++) {
                if (num.charAt(0) == '0' && i > 1) break;
                if (num.charAt(i) == '0' && j > 1) break;
                if(helper(num, i, j)) return true;
            }
        }
        return false;
    }
    
    private boolean helper(String num, int i, int j) {
        String sum;
        Long num1 = Long.parseLong(num.substring(0, i));
        Long num2 = Long.parseLong(num.substring(i, i + j));
        for (int start = i + j; start < num.length(); start += sum.length()) {
            num2 += num1;
            num1 = num2 - num1;
            sum = num2.toString();
            if(!num.startsWith(sum, start)) {
                return false;
            }
        }
        return true;
    }
}

public class Solution {
    public boolean isAdditiveNumber(String num) {
        int n = num.length();
        for (int i = 1; i <= n / 2; ++i) {
            if (num.charAt(0) == '0' && i > 1) return false;
            BigInteger x1 = new BigInteger(num.substring(0, i));
            for (int j = 1; Math.max(j, i) <= n - i - j; ++j) {
                if (num.charAt(i) == '0' && j > 1) break;
                BigInteger x2 = new BigInteger(num.substring(i, i + j));
                if (isValid(x1, x2, j + i, num)) return true;
            }
        }
        return false;
    }
    private boolean isValid(BigInteger x1, BigInteger x2, int start, String num) {
        if (start == num.length()) return true;
        x2 = x2.add(x1);
        x1 = x2.subtract(x1);
        String sum = x2.toString();
        return num.startsWith(sum, start) && isValid(x1, x2, start + sum.length(), num);
    }
}
// Runtime: 8ms