<p>An <strong>additive number</strong> is a string whose digits can form an <strong>additive sequence</strong>.</p>

<p>A valid <strong>additive sequence</strong> should contain <strong>at least</strong> three numbers. Except for the first two numbers, each subsequent number in the sequence must be the sum of the preceding two.</p>

<p>Given a string containing only digits, return <code>true</code> if it is an <strong>additive number</strong> or <code>false</code> otherwise.</p>

<p><strong>Note:</strong> Numbers in the additive sequence <strong>cannot</strong> have leading zeros, so sequence <code>1, 2, 03</code> or <code>1, 02, 3</code> is invalid.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> &quot;112358&quot;
<strong>Output:</strong> true
<strong>Explanation:</strong> 
The digits can form an additive sequence: 1, 1, 2, 3, 5, 8. 
1 + 1 = 2, 1 + 2 = 3, 2 + 3 = 5, 3 + 5 = 8
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> &quot;199100199&quot;
<strong>Output:</strong> true
<strong>Explanation:</strong> 
The additive sequence is: 1, 99, 100, 199.&nbsp;
1 + 99 = 100, 99 + 100 = 199
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= num.length &lt;= 35</code></li>
	<li><code>num</code> consists only of digits.</li>
</ul>

<p>&nbsp;</p>
<p><strong>Follow up:</strong> How would you handle overflow for very large input integers?</p>

class Solution {
public:
    bool help(string n1, string n2, string num, bool found) {
        if (num.size() == 0 && found == true) {
            return true;
        }
        
        long long num1 = stol(n1);
        long long num2 = stol(n2);
        long long num3 = num1 + num2;
        cout<<num1<<" "<<endl;
        string n3 = to_string(num3);

        if (num.substr(0, n3.size()) == n3) {
            return help(n2, n3, num.substr(n3.size()), true);
        }
        
        return false;
    }
    
    bool isAdditiveNumber(string num) {
        if (num.size() <= 2)
            return false;
        
        string n1 = "";
        string n2 = "";
        
        for (int i = 1; i < num.size() - 1; i++) {
            n1 = num.substr(0, i);
            long long num1 = stol(n1);
            if (to_string(num1) != n1) {
                return false;
            }
            
            for (int j = i; j < num.size(); j++) {
                n2 = num.substr(i, j - i + 1);
                long long num2 = stol(n2);
                
                bool found = help(n1, n2, num.substr(j + 1), false);
                
                if (found == true && to_string(num2) == n2)
                    return true;
            }
        }
        return false;
    }
};

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