feat: Add the Sentinel Search algorithm (#883)

* Create sentinel_linear_search.c

This is a search algorithm that uses sentinels.

* Update sentinel_linear_search.c

Added some notes and made some changes

* updating DIRECTORY.md

* Apply suggestions from code review

thx u

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

* Update sentinel_linear_search.c

* Apply suggestions from code review

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>
Co-authored-by: David Leal <halfpacho@gmail.com>

DIRECTORY.md

@@ -374,6 +374,7 @@
     * [Boyer Moore Search](https://github.com/TheAlgorithms/C/blob/master/searching/pattern_search/boyer_moore_search.c)
     * [Naive Search](https://github.com/TheAlgorithms/C/blob/master/searching/pattern_search/naive_search.c)
     * [Rabin Karp Search](https://github.com/TheAlgorithms/C/blob/master/searching/pattern_search/rabin_karp_search.c)
+  * [Sentinel Linear Search](https://github.com/TheAlgorithms/C/blob/master/searching/sentinel_linear_search.c)
   * [Ternary Search](https://github.com/TheAlgorithms/C/blob/master/searching/ternary_search.c)
 
 ## Sorting

searching/sentinel_linear_search.c

@@ -0,0 +1,81 @@
+/**
+ * @file
+ * @brief [Linear Search with Sentinel](https://en.wikipedia.org/wiki/Linear_search#With_a_sentinel) algorithm implementation
+ * @details
+ * This algorithm saves the last element of the array, 
+ * then replaces it with the value to be found and sets it as the sentinel. 
+ * When searching, compares each element with the sentinel. 
+ * If the same, returns the index. If the index is the index of the sentinel, it means it was not found. 
+ * Of course, if the value to be found is the last element, we return the index of the last element.
+ * @author [Regan Yue](https://github.com/ReganYue)
+ * Time Complexity: O(N)
+ */
+
+#include <stdio.h>   /// for IO operations
+#include <assert.h>  /// for assert
+
+/**
+ * @brief Utility function to search for an element in the array and return the index of the element
+ * @details 
+ * The so-called "sentinel" is to use a special value as the boundary key of the array. 
+ * One less judgment statement can be used. 
+ * The purpose is to avoid checking whether the entire array is searched at each step in the search 
+ * process, so as to improve the efficiency of the program.
+ * We can use the last value of the array as the "sentinel", the data storage index i 
+ * starts from 0 and ends at len-1, then the position where the index of arr is n-1 indicates 
+ * that there is no data temporarily, which is the "sentinel" key.
+ * If the last element of the array is equal to the key, directly return the index of the last element. 
+ * Before setting the last element of the array as the key, we hand over the last element of the array to temp for temporary storage. 
+ * Then go to the array to find the key. If the key is found, stop the search, and then compare the found element index with len-1. 
+ * If it is equal, it means it was not found. If it is not equal, it is found.
+ * @param arr this is an array containing elements
+ * @param len this is the number of elements in the array
+ * @param key the value we want to search
+ * @return i if found, otherwise -1 is returned.
+ */
+int sentinel_linear_search( int arr[], int len, int key ){
+	if(key == arr[len-1]){
+		return len-1;
+	}
+	
+	int temp = arr[len-1]; 
+	arr[len-1] = key;
+	
+	int i;
+	for(i=0;arr[len-1]!=arr[i];i++){
+		if(i==len-1){
+			break;
+		}
+	}
+	
+	arr[len-1] = temp;
+	
+	return i != len-1 ? i : -1;
+	 
+}
+
+/**
+ * @brief Self-test implementations
+ * @returns void
+ */
+static void test(){
+	int n,i;
+	n = 5;
+	/* init array */
+	int arr[] = { 1, 2, 2, 6, 99, 100, 999 };
+
+	assert(sentinel_linear_search( arr, n, 1 )==0);
+	assert(sentinel_linear_search( arr, n, 2 )==1);
+	assert(sentinel_linear_search( arr, n, 6 )==3);
+	assert(sentinel_linear_search( arr, n, 101 )==-1);
+	printf("All test cases have successfully passed!\n");
+} 
+
+/**
+ * @brief Main function
+ * @returns 0 on exit
+ */
+int main(){
+	test();  // run self-test implementations
+	return 0;
+}