twoSum.cpp

/*
Given an array of n integers and a number t, determine whether there is a
pair of elements in the array that sums to exactly t and return the indexes
of the first two such elements in the array. If no such pair exist, then
return the string "No such element pair was found."

For example, given the array [2,7,5,2,1], and t = 9, the program
should return the index pair (1,3).
*/

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

using std::cout;
using std::vector;
using std::unordered_map;
using std::pair;

int main()
{
    int target = 13;
    vector<int> inVec {1,7,2,5,9,2,1,8,4,7,6,9,3};
    unordered_map<int,int> umap;
    // begin traversing the vector checking the unordered map for the
    // desired value. If "desired" is not found, then add the current
    // value and its index to the map. If it is found, then return its
    // mapped index and this current index and break the loop.
    int i, desired;
    int EXISTS = 0;
    for (i=0; i<inVec.size(); i++) {
        desired = target - inVec[i];
        if (umap.find(desired) == umap.end()) // not found
            umap.insert(pair(inVec[i],i));
        else {
            cout << "(" << umap[desired] << "," << i << ")\n";
            EXISTS = 1;
            break;
        }
    }
    if (!EXISTS)
        cout << "No such element pair exists.\n";
}