1. Two Sum

TwoSum.kt

//Solves the problem in O(n) time complexity
class Solution {
    fun twoSum(nums: IntArray, target: Int): IntArray {
        //initialize an empty HashMap
        val map = hashMapOf<Int, Int>()
        //and for every element in the array, if it exists in the Map, 
        //check if its complement exists a in the Map or not. If the complement exists then return the indices 
        //of the current element and the complement.
        //Otherwise, put the element in the Map, and move to the next iteration.
        for(i in 0..nums.size - 1){
            if(map.containsKey(target - nums[i])){
                val tmp = map.get(target - nums[i])!!.toInt()
                val ans : IntArray = intArrayOf(tmp,i)
                return ans
            }
            map.put(nums[i],i)
        }
        return intArrayOf()
    }
}