cs: add «15. 3Sum»

URL:	https://leetcode.com/problems/3sum/
Signed-off-by: Matej Focko <me@mfocko.xyz>

cs/3sum.cs

@@ -0,0 +1,32 @@
+public class Solution {
+    public IList<IList<int>> ThreeSum(int[] nums) {
+        Array.Sort(nums);
+
+        var result = new List<IList<int>>();
+        for (var i = 0; i < nums.Length; ++i) {
+            if (i > 0 && nums[i - 1] == nums[i]) {
+                // skip identical numbers
+                continue;
+            }
+
+            var (j, k) = (i + 1, nums.Length - 1);
+            while (j < k) {
+                var sum = nums[i] + nums[j] + nums[k];
+
+                if (sum < 0) {
+                    ++j;
+                } else if (sum == 0) {
+                    result.Add([nums[i], nums[j], nums[k]]);
+
+                    do {
+                        ++j;
+                    } while (nums[j - 1] == nums[j] && j < k);
+                } else {
+                    --k;
+                }
+            }
+        }
+
+        return result;
+    }
+}