Think about the key and value that you will store in the HashMap. Let's consider the example shown above. Iterating across the array [2, 3, 4, 7], at each step let us store the key as target - arr[i], and the value i+1 (since the output is not zero-indexed). Now, if we are at i=1, or arr[i] = 3 and store key = 7 - 3=4 and value = 1 + 1 = 2, then when we get to the next index i = 2, we just need to check if they key arr[i] = 4 exists in the HashMap.
TLDR: Store the difference between the target and the current index in the HashMap so that you can compare subsequent numbers in a single pass!
