Array Hash Map Counting
Learner Level

Implement the twoSumUsingMap method that returns two indices that form the target sum using a hash map.

The input contains an integer array nums, its length size, and an integer target. Your task is to return the indexes of two numbers whose sum is equal to target.

Each input is expected to have one valid pair. The same element cannot be used twice.

For example, in [2,7,11,15] with target 9, values 2 and 7 form the target sum, so the answer is [0,1].

Example 1
Input:
nums (int[]) = [2,7,11,15]
size (int) = 4
target (int) = 9
Return:
(int[]) [0,1]
Example 2
Input:
nums (int[]) = [3,2,4]
size (int) = 3
target (int) = 6
Return:
(int[]) [1,2]
Example 3
Input:
nums (int[]) = [3,3]
size (int) = 2
target (int) = 6
Return:
(int[]) [0,1]

Use a hash map to remember values that have already been visited along with their indexes.

For each current number, calculate the needed complement as target - currentNumber. If the complement already exists in the map, the pair has been found.

If not found, store the current number and its index in the map and continue scanning.

Pseudocode:

function twoSumUsingMap(nums, size, target):
    create empty map valueToIndex
    for i from 0 to size - 1:
        need = target - nums[i]
        if need exists in valueToIndex:
            return [valueToIndex[need], i]
        valueToIndex[nums[i]] = i
    return [-1, -1]
Run your code to see the result.