Unsorted array asked to find two items with sum equal to the given value. Use extra dictionary to store the key and value.

FindTwoItemsWithGivenSum.cs

using System;
using System.Collections.Generic;

class Solution
{
    public static int[] GetIndicesOfItemWeights(int[] arr, int limit)
    {  // [4, 6, 10, 15, 16], 21
      if(arr == null) // false 
        return new int[0]; 
      
      var length = arr.Length; // 5
      var dict = new Dictionary<int, int>(); 
      
      for(int i = 0; i < length; i++)
      {
        var current = arr[i]; // 4, 6, 10, 15
        var search = limit - current; // 17, 15, 11, 6
        
        if(dict.ContainsKey(search)) // true
        {
          return new int[]{i, dict[search]}; // [3, 1] 
        }
        else 
        {
          if(dict.ContainsKey(current)) // 4
          {
            dict[current] = i;
          }
          else             
            dict.Add(current, i); // 4:0, 6:1, 10:2
        }
      }
      
      return new int[0]; 
    }

    static void Main(string[] args)
    {

    }
}

/*
keywords:

array - sorted? 

Ask: find two items with sum equals to given limit
index [i, j] where i > j, arr[i] + arr[j] = limit

Hashmap -> 
  
  [4, 6, 10, 15, 16]
  -> 
  
  4, check hashmap 21 - 4 = 17, if it is in hashmap, 
  put 4: 0 into hashmap
  
  6, look for 21 - 6 = 15 -> hashmap
  put 6: 1 into hashmap
  
  10, 21 - 10 = 11, not found
  put 10:2 into hashmap
  
  15, 21 - 15 = 6, find in hashmap, 1, return [3, 1]
  
    Space complexity: O(n), size of the array
    time complexity:  O(n), n is size of the array
    
    */

I finished the analysis and coding in ten minutes, and then the peer asked me to work on an example, and then I did go over
the example until the pair of indexes are found. Please do not skip the testing the code using an example.