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## Problem

An additive number is a string whose digits can form an additive sequence.

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

Given a string containing only digits, return true if it is an additive number or false otherwise.

Note: Numbers in the additive sequence cannot have leading zeros, so sequence 1, 2, 03 or 1, 02, 3 is invalid.

Example 1:

Input: "112358"
Output: true
Explanation:
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


Example 2:

Input: "199100199"
Output: true
Explanation:
The additive sequence is: 1, 99, 100, 199.
1 + 99 = 100, 99 + 100 = 199


## Code

class Solution {
if(num == null || num.length() <= 2) return false;

for(int i = 1; i <= num.length() / 2; i++){
for(int j = 1; i + j + Math.max(i, j) <= num.length(); j++){
if(isValid(num, i, j)){
return true;
}
}
}

return false;
}

private boolean isValid(String num, int i, int j){
if(num.charAt(0) == '0' && i > 1) return false;
if(num.charAt(i) == '0' && j > 1) return false;

Long sum = (long)0;
Long x = Long.valueOf(num.substring(0, i));
Long y = Long.valueOf(num.substring(i, i + j));

for(int start = i + j; start < num.length(); start += String.valueOf(sum).length()){
sum = x + y;
x = y;
y = sum;

if(!num.startsWith(String.valueOf(sum), start)){
return false;
}
}

return true;
}
}