Day i do not know

1. Two Sum/Solution.cs

@@ -4,10 +4,27 @@ public class Solution
 {
     public int[]? TwoSum(int[] nums, int target)
     {
-        for (int i = 0; i < nums.Length; i++)
-            for (int j = i + 1; j < nums.Length; j++)
-                if (nums[i] + nums[j] == target)
-                    return new int[] { i, j };
+        Dictionary<int, int> dict = new Dictionary<int, int>();
+        int[] halfIndexes = new int[] { -1, -1 };
+        int curHalfIndex = 0;
+        for(int i = 0; i < nums.Length; i++)
+        {
+            if (dict.ContainsKey(nums[i]) == false)
+                dict.Add(nums[i], i);
+            if (nums[i] == target / 2)
+            {
+                halfIndexes[curHalfIndex++] = i;
+                if (target % 2 == 0 && curHalfIndex == 2)
+                    return halfIndexes;
+            }
+        }
+
+        foreach (var kp in dict)
+        {
+            int need = target - kp.Key;
+            if (dict.ContainsKey(need) && (target % 2 == 1 || need != target/2))
+                return new int[] { kp.Value, dict[need] };
+        }
         return null;
     }
 }