feat: add LeetCode problem 540 (#1217)

* feat: add LeetCode problem 540

* feat: Added a description to the LeetCode problem 540

* feat: Added details in the description of the LeetCode problem 540

* feat: Changed a word in @details of the LeetCode problem 540

leetcode/DIRECTORY.md

@@ -91,6 +91,7 @@
 |  485 | [Max Consecutive Ones](https://leetcode.com/problems/max-consecutive-ones)                                                                             | [C](./src/485.c)  | Easy       |
 |  509 | [Fibonacci Number](https://leetcode.com/problems/fibonacci-number)                                                                                     | [C](./src/509.c)  | Easy       |
 |  520 | [Detect Capital](https://leetcode.com/problems/detect-capital)                                                                                         | [C](./src/520.c)  | Easy       |
+|  540 | [Single Element in a Sorted Array](https://leetcode.com/problems/single-element-in-a-sorted-array/)                                                    | [C](./src/540.c)  | Medium     |
 |  561 | [Array Partition](https://leetcode.com/problems/array-partition)                                                                                       | [C](./src/561.c)  | Easy       |
 |  567 | [Permutation in String](https://leetcode.com/problems/permutation-in-string)                                                                           | [C](./src/567.c)  | Medium     |
 |  617 | [Merge Two Binary Trees](https://leetcode.com/problems/merge-two-binary-trees)                                                                         | [C](./src/617.c)  | Easy       |

leetcode/src/540.c

@@ -0,0 +1,32 @@
+/**
+ * Time complexity: O(log n).
+ * Space complexity: O(1).
+ * @details The array has a pattern that consists in of the existing sub-array to 
+ *          the left of the non-repeating number will satisfy the condition that 
+ *          each pair of repeated elements have their first occurrence at the even index
+ *          and their second occurrence at the odd index, and that the sub-array to 
+ *          the right of the non-repeating number will satisfy the condition that 
+ *          each pair of repeated elements have their first occurrence at the odd index
+ *          and their second occurrence at the even index. With this pattern in mind, 
+ *          we can solve the problem using binary search.
+ */
+
+int singleNonDuplicate(int* nums, int numsSize) {
+    int left = 0, right = numsSize - 1;
+    while (left < right) {
+        int mid = (right + left) / 2;
+        if (mid % 2 == 0) {
+            if (nums[mid] == nums[mid + 1])
+                left = mid + 2;
+            else
+                right = mid;
+        }
+        else {
+            if (nums[mid] == nums[mid - 1])
+                left = mid + 1;
+            else
+                right = mid - 1;
+        }
+    }
+    return nums[left];
+}