Find largest d in array such that a + b + c = d in C++

Suppose we have a set of integers. We have to find a number ‘d’, where d = a + b + c, and we have to maximize (a + b + c), all a, b, c, and d are present in the set. The set will hold at least one element, and at max 1000 elements. Each element will be a finite number. If the set is {2, 3, 5, 7, 12}, then 12 is largest d. this can be represented by 2 + 3 + 7

To solve this problem, we can use the hashing technique. We will store the sum of all pairs of (a + b) in the hash table, then traverse through all pairs (c, d) and search (d - c) is present in the table or not. If one match is found, then make sure that no two elements are the same, and no same elements are considered twice.

Example

#include<iostream>
#include<unordered_map>
#include<climits>
using namespace std;
int findElementsInSet(int arr[], int n) {
   unordered_map<int, pair<int, int> > table;
   for (int i = 0; i < n - 1; i++)
      for (int j = i + 1; j < n; j++)
         table[arr[i] + arr[j]] = { i, j };
      int d = INT_MIN;
      for (int i = 0; i < n - 1; i++) {
         for (int j = i + 1; j < n; j++) {
            int abs_diff = abs(arr[i] - arr[j]);
            if (table.find(abs_diff) != table.end()) {
               pair<int, int> p = table[abs_diff];
               if (p.first != i && p.first != j && p.second != i && p.second != j) d = max(d, max(arr[i], arr[j]));
            }
         }
      }
   return d;
}
int main() {
   int arr[] = { 2, 3, 5, 7, 12 };
   int n = sizeof(arr) / sizeof(arr[0]);
   int res = findElementsInSet(arr, n);
   if (res == INT_MIN)
      cout << "Cannot find the value of d";
   else
      cout << "Max value of d is " << res;
}

Output

Max value of d is 12