Same as two sum.
x - y = 7 => y = x - 7
y - x = 7 => y = x + 7
For each x in A
see if we have y using both the equations.
int Solution::solve(vector<int> &A, int B) {
unordered_set<int> s;
for(int x: A)
if(s.count(B + x) || s.count(x - B))
return true;
else
s.insert(x);
return false;
}