feat: add sentinel search algorithm

Search/QuickSelectSearch.js

@@ -21,6 +21,7 @@ export function quickSelectSearch(array, k) {
   while (from < to) {
     let left = from
     let right = to
+
     const pivot = array[Math.ceil((left + right) * 0.5)]
 
     while (left < right) {

Search/SentinelSearch.js

@@ -0,0 +1,35 @@
+/**
+ * @description 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 : O(n)
+ * Auxiliary Space: O(1)
+ */
+
+export const sentinelSearch = (array, target) => {
+  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 = 0
+  while (array[index] !== target) index++
+
+  // Put the last element back
+  array[arrayLength - 1] = last
+
+  if (index < arrayLength - 1 || array[arrayLength - 1] === target) return index
+  return null
+}

Search/TernarySearch.js

@@ -12,38 +12,33 @@
  */
 
 function ternarySearchRecursive(arr, key, low = 0, high = arr.length - 1) {
-  if (high >= low) {
-    // find the mid1 and mid2
-    const mid1 = Math.floor(low + (high - low) / 3)
-    const mid2 = Math.floor(high - (high - low) / 3)
+  // if low > high => we have searched the whole array without finding the item
+  if (low > high) return -1
 
-    // check if key is found at any mid
-    if (arr[mid1] === key) {
-      // return index of key if found
-      return mid1
-    }
-    if (arr[mid2] === key) {
-      // return index of key if found
-      return mid2
-    }
+  // find the mid1 and mid2
+  const mid1 = Math.floor(low + (high - low) / 3)
+  const mid2 = Math.floor(high - (high - low) / 3)
 
-    // since the key is not found at mid,
-    // check in which region it is present
-    // and repeat the Search operation
-    // in that region
-    if (key < arr[mid1]) {
-      // the key lies in between low and mid1
-      return ternarySearchRecursive(arr, key, low, mid1 - 1)
-    } else if (key > arr[mid2]) {
-      // the key lies in between mid2 and high
-      return ternarySearchRecursive(arr, key, mid2 + 1, high)
-    } else {
-      // the key lies in between mid1 and mid2
-      return ternarySearchRecursive(arr, key, mid1 + 1, mid2 - 1)
-    }
+  // check if key is found at any mid
+  // return index of key if found
+  if (arr[mid1] === key) return mid1
+
+  // return index of key if found
+  if (arr[mid2] === key) return mid2
+
+  // since the key is not found at mid,
+  // check in which region it is present
+  // and repeat the Search operation
+  // in that region
+  if (key < arr[mid1]) {
+    // the key lies in between low and mid1
+    return ternarySearchRecursive(arr, key, low, mid1 - 1)
+  } else if (key > arr[mid2]) {
+    // the key lies in between mid2 and high
+    return ternarySearchRecursive(arr, key, mid2 + 1, high)
   } else {
-    // if low > high => we have searched the whole array without finding the item
-    return -1
+    // the key lies in between mid1 and mid2
+    return ternarySearchRecursive(arr, key, mid1 + 1, mid2 - 1)
   }
 }
 
@@ -54,14 +49,11 @@ function ternarySearchIterative(arr, key, low = 0, high = arr.length - 1) {
     const mid2 = Math.floor(high - (high - low) / 3)
 
     // check if key is found at any mid
-    if (arr[mid1] === key) {
-      // return index of key if found
-      return mid1
-    }
-    if (arr[mid2] === key) {
-      // return index of key if found
-      return mid2
-    }
+    // return index of key if found
+    if (arr[mid1] === key) return mid1
+
+    // return index of key if found
+    if (arr[mid2] === key) return mid2
 
     // since the key is not found at mid,
     // check in which region it is present
@@ -75,8 +67,7 @@ function ternarySearchIterative(arr, key, low = 0, high = arr.length - 1) {
       low = mid2 + 1
     } else {
       // the key lies in between mid1 and mid2
-      low = mid1 + 1
-      high = mid2 - 1
+      ;[low, high] = [mid1 + 1, mid2 - 1]
     }
   }
   // the key was not found

Search/test/SentinelSearch.test.js

@@ -0,0 +1,13 @@
+import { sentinelSearch } from '../SentinelSearch'
+
+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, target, index) => {
+    expect(sentinelSearch(array, target)).toStrictEqual(index)
+  })
+})

Search/test/TernarySearch.test.js

@@ -29,13 +29,13 @@ test('should return the index of a number in an array of numbers:', () => {
 test('should return the index of a string in an array of strings:', () => {
   const indexNumber = ternarySearchRecursive(
     ['Ali', 'Cathrynli', 'Josuke', 'Thomas'],
-    'Cathrynli'
+    'Josuke'
   )
-  expect(indexNumber).toBe(1)
+  expect(indexNumber).toBe(2)
 })
 
 test('should return the index of a string in an array of strings:', () => {
-  const indexNumber = ternarySearchRecursive(
+  const indexNumber = ternarySearchIterative(
     ['Ali', 'Cathrynli', 'Josuke', 'Thomas'],
     'Josuke'
   )
@@ -49,3 +49,11 @@ test('should return the index of a string in an array of strings:', () => {
   )
   expect(indexNumber).toBe(-1)
 })
+
+test('should return the index of a string in an array of strings:', () => {
+  const indexNumber = ternarySearchIterative(
+    ['Ali', 'Cathrynli', 'Josuke', 'Thomas'],
+    'Angela'
+  )
+  expect(indexNumber).toBe(-1)
+})