Sum of Two Values

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Problem Statement Hints Try it yourself Solution Solution Explanation Runtime Complexity Memory Complexity Solution Breakdown

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Sum of Two Values

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Oct 15, 2025

bool find_sum_of_two(vector<int>& A, int val) {
  unordered_set<int> found_values;
  for (int& a : A) {
    if (found_values.find(val - a) != found_values.end()) {
      return true;
    }
    found_values.insert(a);
  }
  return false;
}
int main() {
  vector<int> v = {5, 7, 1, 2, 8, 4, 3};
  vector<int> test = {3, 20, 1, 2, 7};
  for(int i=0; i<test.size(); i++){
    bool output = find_sum_of_two(v, test[i]);
    cout << "find_sum_of_two(v, " << test[i] << ") = " << (output ? "true" : "false") << endl;
  }
  return 0;
}

Solution Explanation#

Runtime Complexity#

The runtime complexity of this solution is linear, O(n).

Memory Complexity#

The memory complexity of this solution is linear, O(n).

Solution Breakdown#

In this solution, you can use the following algorithm to find a pair that add up to the target (say val).

Scan the whole array once and store visited elements in a hash set.
During scan, for every element e in the array, we check if val - e is present in the hash set i.e. val - e is already visited.
- If val - e is found in the hash set, it means there is a pair (e, val - e) in array whose sum is equal to the given val.
- If we have exhausted all elements in the array and didn’t find any such pair, the function will return false.