leetcode C# problem

create two-sum.cs

@@ -0,0 +1,12 @@
+public class Solution {
+    public int[] TwoSum(int[] nums, int target) {
+        Dictionary<int, int> lookup = new Dictionary<int, int>();
+        for (var i = 0; i < nums.Length; i++) {
+            if (lookup.ContainsKey(target - nums[i])) {
+                return new int [] { lookup[target - nums[i]], i };
+            }
+            lookup[nums[i]] = i;
+        }
+        return new int[] { };
+    }
+}

sum-with-multiplicity

@@ -0,0 +1,25 @@
+# Time:  O(n^2), n is the number of disctinct A[i]
+# Space: O(n)
+
+import collections
+import itertools
+
+
+class Solution(object):
+    def threeSumMulti(self, A, target):
+        """
+        :type A: List[int]
+        :type target: int
+        :rtype: int
+        """
+        count = collections.Counter(A)
+        result = 0
+        for i, j in itertools.combinations_with_replacement(count, 2):
+            k = target - i - j
+            if i == j == k:
+                result += count[i] * (count[i]-1) * (count[i]-2) // 6
+            elif i == j != k:
+                result += count[i] * (count[i]-1) // 2 * count[k]
+            elif max(i, j) < k:
+                result += count[i] * count[j] * count[k]
+        return result % (10**9 + 7)