feat: add solution for the 3Sum Closest problem (#16) (#1216)

* feat: add solution for the 3Sum Closest problem (#16)

* fix: Update formatting

* fix: update compare function to avoid overflow in generic case

* chore: apply suggestions from code review

---------

Co-authored-by: David Leal <halfpacho@gmail.com>

leetcode/DIRECTORY.md

@@ -18,6 +18,7 @@
 |   12 | [Integer to Roman](https://leetcode.com/problems/integer-to-roman)                                                                                     | [C](./src/12.c)   | Medium     |
 |   13 | [Roman to Integer](https://leetcode.com/problems/roman-to-integer)                                                                                     | [C](./src/13.c)   | Easy       |
 |   14 | [Longest Common Prefix](https://leetcode.com/problems/longest-common-prefix)                                                                           | [C](./src/14.c)   | Easy       |
+|   16 | [3Sum Closest](https://leetcode.com/problems/3sum-closest)                                                                                             | [C](./src/16.c)   | Medium     |
 |   20 | [Valid Parentheses](https://leetcode.com/problems/valid-parentheses)                                                                                   | [C](./src/20.c)   | Easy       |
 |   21 | [Merge Two Sorted Lists](https://leetcode.com/problems/merge-two-sorted-lists)                                                                         | [C](./src/21.c)   | Easy       |
 |   24 | [Swap Nodes in Pairs](https://leetcode.com/problems/swap-nodes-in-pairs)                                                                               | [C](./src/24.c)   | Medium     |

leetcode/src/16.c

@@ -0,0 +1,30 @@
+#include <stdlib.h> // for qsort()
+
+int cmp(const void* a, const void* b) {
+    const int *A = a, *B = b;
+    return (*A > *B) - (*A < *B);
+}
+
+int threeSumClosest(int* nums, int nums_size, int target) {
+    int i, j, k, result, sum3;
+    qsort(nums, nums_size, sizeof(int), cmp);
+    result = nums[0] + nums[1] + nums[2];
+    for (i = 0; i < nums_size - 2; i++) {
+        j = i + 1;
+        k = nums_size - 1;
+        while (j < k) {
+            sum3 = nums[i] + nums[j] + nums[k];
+            if (abs(target - sum3) < abs(target - result)) {
+                result = sum3;
+            }
+            if (sum3 < target) {
+                j++;
+            } else if (sum3 > target) {
+                k--;
+            } else {
+                return sum3;
+            }
+        }
+    }
+    return result;
+}