# Sentinel Linear Search

• Difficulty Level : Easy
• Last Updated : 04 Aug, 2021

Sentinel Linear Search as the name suggests is a type of Linear Search where the number of comparisons is reduced as compared to a traditional linear search. When a linear search is performed on an array of size N then in the worst case a total of N comparisons are made when the element to be searched is compared to all the elements of the array and (N + 1) comparisons are made for the index of the element to be compared so that the index is not out of bounds of the array which can be reduced in a Sentinel Linear Search.
In this search, the last element of the array is replaced with the element to be searched and then the linear search is performed on the array without checking whether the current index is inside the index range of the array or not because the element to be searched will definitely be found inside the array even if it was not present in the original array since the last element got replaced with it. So, the index to be checked will never be out of bounds of the array. The number of comparisons in the worst case here will be (N + 2).
Examples:

Input: arr[] = {10, 20, 180, 30, 60, 50, 110, 100, 70}, x = 180
Output: 180 is present at index 2
Input: arr[] = {10, 20, 180, 30, 60, 50, 110, 100, 70}, x = 90

Below is the implementation of the above approach:

## C++

 `// C++ implementation of the approach``#include ``using` `namespace` `std;` `// Function to search x in the given array``void` `sentinelSearch(``int` `arr[], ``int` `n, ``int` `key)``{` `    ``// Last element of the array``    ``int` `last = arr[n - 1];` `    ``// Element to be searched is``    ``// placed at the last index``    ``arr[n - 1] = key;``    ``int` `i = 0;` `    ``while` `(arr[i] != key)``        ``i++;` `    ``// Put the last element back``    ``arr[n - 1] = last;` `    ``if` `((i < n - 1) || (arr[n - 1] == key))``        ``cout << key << ``" is present at index "` `<< i;``    ``else``        ``cout << ``"Element Not found"``;``}` `// Driver code``int` `main()``{``    ``int` `arr[] = { 10, 20, 180, 30, 60, 50, 110, 100, 70 };``    ``int` `n = ``sizeof``(arr) / ``sizeof``(arr[0]);``    ``int` `key = 180;` `    ``sentinelSearch(arr, n, key);` `    ``return` `0;``}``// This code is contributed by Mandeep Dalavi`

## Java

 `// Java implementation of the approach``class` `GFG {` `    ``// Function to search x in the given array``    ``static` `void` `sentinelSearch(``int` `arr[], ``int` `n, ``int` `key)``    ``{` `        ``// Last element of the array``        ``int` `last = arr[n - ``1``];` `        ``// Element to be searched is``        ``// placed at the last index``        ``arr[n - ``1``] = key;``        ``int` `i = ``0``;` `        ``while` `(arr[i] != key)``            ``i++;` `        ``// Put the last element back``        ``arr[n - ``1``] = last;` `        ``if` `((i < n - ``1``) || (arr[n - ``1``] == key))``            ``System.out.println(key + ``" is present at index "``                               ``+ i);``        ``else``            ``System.out.println(``"Element Not found"``);``    ``}` `    ``// Driver code``    ``public` `static` `void` `main(String[] args)``    ``{``        ``int` `arr[]``            ``= { ``10``, ``20``, ``180``, ``30``, ``60``, ``50``, ``110``, ``100``, ``70` `};``        ``int` `n = arr.length;``        ``int` `key = ``180``;` `        ``sentinelSearch(arr, n, key);``    ``}``}` `// This code is contributed by Ankit Rai, Mandeep Dalavi`

## Python3

 `# Python3 implementation of the approach``# Function to search key in the given array`  `def` `sentinelSearch(arr, n, key):` `    ``# Last element of the array``    ``last ``=` `arr[n ``-` `1``]` `    ``# Element to be searched is``    ``# placed at the last index``    ``arr[n ``-` `1``] ``=` `key``    ``i ``=` `0` `    ``while` `(arr[i] !``=` `key):``        ``i ``+``=` `1` `    ``# Put the last element back``    ``arr[n ``-` `1``] ``=` `last` `    ``if` `((i < n ``-` `1``) ``or` `(arr[n ``-` `1``] ``=``=` `key)):``        ``print``(key, ``"is present at index"``, i)``    ``else``:``        ``print``(``"Element Not found"``)`  `# Driver code``arr ``=` `[``10``, ``20``, ``180``, ``30``, ``60``, ``50``, ``110``, ``100``, ``70``]``n ``=` `len``(arr)``key ``=` `180` `sentinelSearch(arr, n, key)` `# This code is contributed by divyamohan123, Mandeep Dalavi`

## C#

 `// C# implementation of the approach``using` `System;` `class` `GFG {` `    ``// Function to search x in the given array``    ``static` `void` `sentinelSearch(``int``[] arr, ``int` `n, ``int` `key)``    ``{` `        ``// Last element of the array``        ``int` `last = arr[n - 1];` `        ``// Element to be searched is``        ``// placed at the last index``        ``arr[n - 1] = key;``        ``int` `i = 0;` `        ``while` `(arr[i] != key)``            ``i++;` `        ``// Put the last element back``        ``arr[n - 1] = last;` `        ``if` `((i < n - 1) || (arr[n - 1] == key))``            ``Console.WriteLine(key + ``" is present"``                              ``+ ``" at index "` `+ i);``        ``else``            ``Console.WriteLine(``"Element Not found"``);``    ``}` `    ``// Driver code``    ``public` `static` `void` `Main()``    ``{``        ``int``[] arr``            ``= { 10, 20, 180, 30, 60, 50, 110, 100, 70 };``        ``int` `n = arr.Length;``        ``int` `key = 180;` `        ``sentinelSearch(arr, n, key);``    ``}``}` `// This code is contributed by Mohit kumar, Mandeep Dalavi`

## Javascript

 ``

Output:

`180 is present at index 2`

Time Complexity: O(N)
Auxiliary Space: O(1)

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