feat: add csharp solution to lc problem: No.0016 (#4029)

Author: djvelimir

solution/0000-0099/0016.3Sum Closest/README.md

@@ -242,6 +242,36 @@ var threeSumClosest = function (nums, target) {
 };
 ```
 
+#### C#
+
+```cs
+public class Solution {
+    public int ThreeSumClosest(int[] nums, int target) {
+        Array.Sort(nums);
+        int ans = 1 << 30;
+        int n = nums.Length;
+        for (int i = 0; i < n; ++i) {
+            int j = i + 1, k = n - 1;
+            while (j < k) {
+                int t = nums[i] + nums[j] + nums[k];
+                if (t == target) {
+                    return t;
+                }
+                if (Math.Abs(t - target) < Math.Abs(ans - target)) {
+                    ans = t;
+                }
+                if (t > target) {
+                    --k;
+                } else {
+                    ++j;
+                }
+            }
+        }
+        return ans;
+    }
+}
+```
+
 #### PHP
 
 ```php

solution/0000-0099/0016.3Sum Closest/README_EN.md

@@ -241,6 +241,36 @@ var threeSumClosest = function (nums, target) {
 };
 ```
 
+#### C#
+
+```cs
+public class Solution {
+    public int ThreeSumClosest(int[] nums, int target) {
+        Array.Sort(nums);
+        int ans = 1 << 30;
+        int n = nums.Length;
+        for (int i = 0; i < n; ++i) {
+            int j = i + 1, k = n - 1;
+            while (j < k) {
+                int t = nums[i] + nums[j] + nums[k];
+                if (t == target) {
+                    return t;
+                }
+                if (Math.Abs(t - target) < Math.Abs(ans - target)) {
+                    ans = t;
+                }
+                if (t > target) {
+                    --k;
+                } else {
+                    ++j;
+                }
+            }
+        }
+        return ans;
+    }
+}
+```
+
 #### PHP
 
 ```php

solution/0000-0099/0016.3Sum Closest/Solution.cs

@@ -0,0 +1,25 @@
+public class Solution {
+    public int ThreeSumClosest(int[] nums, int target) {
+        Array.Sort(nums);
+        int ans = 1 << 30;
+        int n = nums.Length;
+        for (int i = 0; i < n; ++i) {
+            int j = i + 1, k = n - 1;
+            while (j < k) {
+                int t = nums[i] + nums[j] + nums[k];
+                if (t == target) {
+                    return t;
+                }
+                if (Math.Abs(t - target) < Math.Abs(ans - target)) {
+                    ans = t;
+                }
+                if (t > target) {
+                    --k;
+                } else {
+                    ++j;
+                }
+            }
+        }
+        return ans;
+    }
+}