twoSum.cpp

/*
Given an array of integers nums and an integer target, return indices of the two numbers such that they add up to target.

You may assume that each input would have exactly one solution, and you may not use the same element twice.

You can return the answer in any order.

 

Example 1:

Input: nums = [2,7,11,15], target = 9
Output: [0,1]
Explanation: Because nums[0] + nums[1] == 9, we return [0, 1].
Example 2:

Input: nums = [3,2,4], target = 6
Output: [1,2]
Example 3:

Input: nums = [3,3], target = 6
Output: [0,1]

*/


#include <iostream>
#include <vector>
#include <unordered_map>

using namespace std;

//Brute force
vector<int> twoSumBrute(vector<int>& nums, int target){

    vector<int> target_indices;

    for(int i = 0; i < nums.size(); i++){
        for(int j = i + 1; j < nums.size(); j++){
            if(nums.at(i) + nums.at(j) == target){
                target_indices.push_back(i);
                target_indices.push_back(j);
                break;
            }
        }
    }

    return target_indices;
}

vector<int> twoSumFast(vector<int>& nums, int target){
    vector<int> target_indices;

    unordered_map<int, int> hash_table;
    for(int i = 0; i < nums.size(); i++){
        int second_integer = target - nums.at(i);

        if(hash_table.find(second_integer) != hash_table.end()){
            target_indices.push_back(i);
            target_indices.push_back(hash_table.find(second_integer)->second);
            break;
        }
        else{
            hash_table[nums.at(i)] = i;
        }
    }


    return target_indices;
}

int main(){

    int numInArray = 0;
    int target;
    vector<int> key(2);

    cout << "Please enter the number integers in the array: ";
    cin >> numInArray;

    vector<int> array(numInArray);
    cout << "Enter " << numInArray << " elements:" << endl;
    for(int i = 0; i < numInArray; ++i){
        cin >> array[i];
    }

    cout << "Please enter target value: " << endl;
    cin >> target;

    key = twoSumBrute(array, target);


        cout << "[" << key[0] << "," << key[1] << "]" << endl;
    




    return 0;
}