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String with additive sequence
• Difficulty Level : Hard
• Last Updated : 27 Mar, 2017

Given a string, the task is to find whether it contains an additive sequence or not. A string contains an additive sequence if its digits can make a sequence of numbers in which every number is addition of previous two numbers. A valid string should contain at least three digit to make one additive sequence.

Examples:

```Input : s = “235813”
Output : true
2 + 3 = 5, 3 + 5 = 8, 5 + 8 = 13

Input : s = “199100199”
Output : true
1 + 99 = 100, 99 + 100 = 199

Input : s = “12345678”
Output : false
```

## Recommended: Please solve it on “PRACTICE” first, before moving on to the solution.

This problem can be solved recursively, note that number of digits in added value can’t be smaller than digits in any of its operand that is why we will loop till (length of string)/2 for first number and (length of string – first number’s length)/ 2 for second number to ignore invalid result.
Next thing to note is, first and second number can’t start with 0, which is checked in below code by isValid method. When we call recursively, we check that sum of first and second number is exactly equal to rest of string. If yes then direct return the result else check that sum string is prefix of rest of string or not, If yes then call recursively with second number, sum string and rest of string after removing sum string from rest of string and if sum string is not prefix of rest of string then no solution in available.

Below is C++ implementation.

 `// C++ program to check whether a string``// makes an additive sequence or not``#include ``using` `namespace` `std;`` ` `// Checks whether num is valid or not, by``// checking first character and size``bool` `isValid(string num)``{``    ``if` `(num.size() > 1 && num == ``'0'``)``        ``return` `false``;``    ``return` `true``;``}`` ` `// returns int value at pos string, if pos is``// out of bound then returns 0``int` `val(string a, ``int` `pos)``{``    ``if` `(pos >= a.length())``        ``return` `0;`` ` `    ``//  converting character to integer``    ``return` `(a[pos] - ``'0'``);``}`` ` `// add two number in string form and return``// result as a string``string addString(string a, string b)``{``    ``string sum = ``""``;``    ``int` `i = a.length() - 1;``    ``int` `j = b.length() - 1;``    ``int` `carry = 0;`` ` `    ``//  loop untill both string get processed``    ``while` `(i >= 0 || j >= 0)``    ``{``        ``int` `t = val(a, i) + val(b, j) + carry;``        ``sum += (t % 10 + ``'0'``);``        ``carry = t / 10;``        ``i--;    j--;``    ``}``    ``if` `(carry)``        ``sum += (carry + ``'0'``);``    ``reverse(sum.begin(), sum.end());``    ``return` `sum;``}`` ` `//  Recursive method to check c = a + b``bool` `checkAddition(list& res, string a,``                             ``string b, string c)``{``    ``//  both first and second number should be valid``    ``if` `(!isValid(a) || !isValid(b))``        ``return` `false``;``    ``string sum = addString(a, b);`` ` `    ``//  if sum is same as c then direct return``    ``if` `(sum == c)``    ``{``        ``res.push_back(sum);``        ``return` `true``;``    ``}`` ` `    ``/*  if sum size is greater than c, then no``        ``possible sequence further OR if c is not``        ``prefix of sum string, then no possible``        ``sequence further  */``    ``if` `(c.size() <= sum.size() ||``        ``sum != c.substr(0, sum.size()))``        ``return` `false``;``    ``else``    ``{``        ``res.push_back(sum);``         ` `        ``//  next recursive call will have b as first``        ``//  number, sum as second number and string``        ``//  c as third number after removing prefix``        ``//  sum string from c``        ``return` `checkAddition(res, b, sum,``                             ``c.substr(sum.size()));``    ``}``}`` ` `//  Method returns additive sequence from string as``// a list``list additiveSequence(string num)``{``    ``list res;``    ``int` `l = num.length();`` ` `    ``// loop untill l/2 only, because if first``    ``// number is larger,then no possible sequence``    ``// later``    ``for` `(``int` `i = 1; i <= l/2; i++)``    ``{``        ``for` `(``int` `j = 1; j <= (l - i)/2; j++)``        ``{``            ``if` `(checkAddition(res, num.substr(0, i),``                              ``num.substr(i, j),``                              ``num.substr(i + j)))``            ``{``                ``// adding first and second number at``                ``// front of result list``                ``res.push_front(num.substr(i, j));``                ``res.push_front(num.substr(0, i));``                ``return` `res;``            ``}``        ``}``    ``}`` ` `    ``// If code execution reaches here, then string``    ``// doesn't have any additive sequence``    ``res.clear();``    ``return` `res;``}`` ` `//  Method to print result list``void` `printResult(list res)``{``    ``for` `(``auto` `it = res.begin(); it != res.end(); it++)``        ``cout << *it << ``" "``;``    ``cout << endl;``}`` ` `//  Driver code to test above methods``int` `main()``{``    ``string num = ``"235813"``;``    ``list res = additiveSequence(num);``    ``printResult(res);`` ` `    ``num = ``"199100199"``;``    ``res = additiveSequence(num);``    ``printResult(res);``    ``return` `0;``}`

Output:

```2 3 5 8 13
1 99 100 199
```

This article is contributed by Utkarsh Trivedi. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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