length of longest fibonaaci sequence

/*
A sequence X_1, X_2, ..., X_n is fibonacci-like if:

n >= 3
X_i + X_{i+1} = X_{i+2} for all i + 2 <= n
Given a strictly increasing array A of positive integers forming a sequence, find the length of the longest fibonacci-like subsequence of A. 
If one does not exist, return 0.

(Recall that a subsequence is derived from another sequence A by deleting any number of elements (including none) from A, without changing the
order of the remaining elements.  For example, [3, 5, 8] is a subsequence of [3, 4, 5, 6, 7, 8].)

 

Example 1:

Input: [1,2,3,4,5,6,7,8]
Output: 5
Explanation:
The longest subsequence that is fibonacci-like: [1,2,3,5,8].
Example 2:

Input: [1,3,7,11,12,14,18]
Output: 3
Explanation:
The longest subsequence that is fibonacci-like:
[1,11,12], [3,11,14] or [7,11,18].
*/

class Solution {
public:
    int lenLongestFibSubseq(vector<int>& A) {
        int res=0, n=A.size();
        unordered_set<int> st(A.begin(), A.end());
        for(int i=0; i<n; i++){
             for(int j=i+1; j<n; j++){
                int a=A[i], b=A[j] , c=2;                  
                 while(st.count(a+b)){
                              b=a+b;  a=b-a; ++c;                    

                 }
                 
res=max(res,c);
             }           
 
        }       
 return (res > 2)? res: 0;
 
    }
};