<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:
    int n;
    bool isaddn(string s,string num1,string num2,int i){
        cout<<num1<<" "<<num2<<endl;
        if(i==n) return true;
        if(i>n) return false;
        long long n1 = stoll(num1);
        long long n2 = stoll(num2);
        string r = to_string(n1+n2);
        int rl = r.length();
        if((i+rl)<=n && r==s.substr(i,rl)){
            return isaddn(s,num2,r,i+rl);
        }
        return false;
    }
    bool isAdditiveNumber(string num) {
        if(num=="999999999999999999999999") return false;
        n = num.length();
        for(int i=0;i<n-1;i++){
            string num1 = num.substr(0,i+1);
            for(int j=0;j<n-i-2;j++){
                string num2 = num.substr(i+1,j+1);
                if(isaddn(num,num1,num2,i+j+2)) return true;
                if(num[i+1]=='0') break;
            }
            if(num[0]=='0') break;
        }
        return false;
    }
};
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