feat: Added Sentinel Search (TheAlgorithms#163)

* feat: added Sentinel Search

* return type to null if element not found

Author: anshvert

search/sentinel_search.ts

@@ -0,0 +1,42 @@
+/**
+ * @function sentinelSearch
+ * @description Sentinel search algorithm for array. 
+ * 
+ * Sentinel linear search is a variation of the standard linear search algorithm used to 
+ * find a target value in an array or list. The basic idea behind this algorithm is to add a 
+ * sentinel value at the end of the array which is equal to the target value we are looking for.
+ * This helps to avoid checking the array boundary condition during each iteration of the loop, 
+ * as the sentinel value acts as a stopper for the loop.
+ * 
+ * @param {number[]} array - sorted list of numbers
+ * @param {number} target - target number to search for
+ * @return {number|null} - index of the target number in the list, or null if not found
+ * @see [SentinelSearch](https://www.geeksforgeeks.org/sentinel-linear-search/)
+ * @example sentinelSearch([1,2,3], 2) => 1
+ * @example sentinelSearch([4,5,6], 2) => null
+ * @complexity_analysis
+ * Time Complexity :
+ * Worst Case -> The time complexity of the Sentinel Linear Search algorithm is O(n) in the worst case.
+ * Best Case -> In the best case, when the key is found in the first iteration, the time complexity will be O(1).
+ * Average Case -> However, the average time complexity is still O(n).
+ * Auxiliary Space: O(1)
+ */
+
+export const sentinelSearch = (array: number[], target: number): number|null => {
+    const arrayLength = array.length
+    if (arrayLength === 0) return null;
+
+    // Element to be searched is placed at the last index
+    const last = array[arrayLength-1]
+    array[arrayLength-1] = target
+
+    let index: number = 0
+    while (array[index] !== target) index += 1
+
+    // Put the last element back
+    array[arrayLength-1] = last
+    
+    if ((index < arrayLength - 1) || (array[arrayLength - 1] === target))
+        return index
+    return null
+}

search/test/sentinel_search.test.ts

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+import { sentinelSearch } from "../sentinel_search";
+
+describe("Sentinel search", () => {
+  test.each([
+    [['o', 'b', 'c'], 'c', 2],
+    [[1, 2, 3, 4, 5], 4, 3],
+    [['s', 't', 'r', 'i', 'n', 'g'], 'a', null],
+    [['1','2','3'],'1',0],
+    [['4','e','6','10'],4,null]
+  ])(
+    "of %o , searching for %o, expected %i",
+    (array: any[], target: any, index: number|null) => {
+      expect(sentinelSearch(array, target)).toStrictEqual(index)
+    },
+  );
+});