C++ twoSum. Optimize memory usage - c++

Question

I am solving a twoSum problem.
Steps:
Read an input file with a following template:
7
1 7 3 4 7 9
First line is the target number, second line is a number sequence.
Numbers can be in range 0 < N < 999999999
If the sum of two numbers from the number sequence equals the target number, I write "1" to the output file.
If there are no numbers the sum of which equals the target number then I write "0" to the output file.
I need to optimize memory usage in my code. How can I do that?
#include <iostream>
#include <fstream>
#include <sstream>
#include <vector>
#include <unordered_map>
#include <string>
#include <algorithm>
using namespace std;
int main() {
ifstream f1;
vector<int> nums;
string line;
string source;
//Open input file and read it to a string
f1.open("input.txt");
while (getline(f1, line, '\n')) {
source+=line + " ";
} f1.close();
//Parse the string into an int vector
stringstream parser(source);
int num;
while (parser >> num) { nums.push_back(num); }
//Clear used strings
line = "";
source = "";
//Get target number
int target = nums.at(0);
//Get number sequence
nums.erase(nums.begin());
bool flag = false;
//Check number sequence for two numbers sum of which equals the target number
unordered_map<int, int> mp;
for(int i=0;i<nums.size();i++){
if(mp.count(nums[i])==1){
flag = true;
break;}
mp[target-nums[i]]=i;
}
//Write the result into the output file
ofstream f2;
f2.open("output.txt");
f2 << flag;
f2.close();
}

Answer 1

There are a couple of things you can do to minimise memory usage here. First up, you don't need to read entire contents of the file into std::string. You can read directly into std::vector, or better still read the file contents into a single int variable and process the numbers as you go. Another thing: you do not need to use a std::unordered_map, because presence of the key is the only thing you are really interested in, so std::unordered_set is sufficient. Below a simple solution making use of that suggestions:
#include <fstream>
#include <unordered_set>
int main() {
std::ifstream input {"input.txt"};
int target;
input >> target;
int current_number;
bool found = false;
std::unordered_set<int> observed_numbers;
while (input >> current_number) {
if (observed_numbers.count(target - current_number) > 0) {
found = true;
break;
}
observed_numbers.insert(current_number);
}
std::ofstream output {"output.txt"};
output << found;
}