Binary Search

Difficulty: Easy

Understanding The Problem

Problem Description

Given a sorted integer array arr[] of n elements and a target value k , write a program to search k in arr[] .

Problem Note

If k exists, then return its index, otherwise, return -1.
You may assume that all elements in arr[] are unique.

Example 1

Input: arr[] = [1, 5, 6, 7, 9, 10, 50], k= 9
Output: 4
Explanation: 9 exists in arr[] and its index is 4.

Example 2

Input: arr[] = [1, 5, 6, 7, 9, 10, 50], target = 20
Output: -1
Explanation: 20 does not exist in arr[] so return -1.

You may try to solve the problem here.

Solution

We need to return the position of an element that needs to be searched in a sorted array.

The question clearly needs us to perform Binary Search. This searching technique works on the sorted arrays.

The binary search begins by comparing an element in the middle of the array with the target value. If the target value matches the element, its position in the array is returned. If the target value is less than the element, the search continues in the lower half of the array. If the target value is greater than the element, the search continues in the upper half of the array.

By doing this, the algorithm eliminates the half in which the target value cannot lie in each iteration.

This searching technique is far more efficient than the linear search. Look at the example below.

Solution Steps

Set L to 0 and R to n-1.
If L > R, the search terminates as unsuccessful.
Set m (the position of the middle element) to the floor of (l + r) / 2
If A[m] < T, set L to m+1 and go to step 2.
If A[m] > T, set R to m-1 and go to step 2.
If A[m]=T, the search is done and return m.

Pseudo Code

Recursive Binary Search

int binarySearch(int arr[], int l, int r, int T) { 
    if (l <= r) { 
        int mid = l + (r - l) / 2
        if (arr[mid] == T) 
            return mid
        if (arr[mid] > T) 
            return binarySearch(arr, l, mid - 1, T)
        return binarySearch(arr, mid + 1, r, T) 
    } 
    return -1
}

Complexity Analysis

Time Complexity: O(log n)

Space Complexity: O(log n), considering recursion stack space

Iterative Binary Search

int binarySearch(int arr[], int l, int r, int T) 
{ 
    while (l <= r) { 
        int m = l + (r - l) / 2
        if (arr[m] == T) 
            return m
        if (arr[m] < T) 
            l = m + 1 
        else
            r = m - 1
    } 
    return -1
}

Complexity Analysis

Time Complexity: O(log n)

Space Complexity: O(1)

Here’s a table showing the base-2 logarithms of various values of n :

Critical Ideas To Think

Can we replace while(l≤r) with while(l<r) ? If not, then why?
How did we make sure than if the target < arr[m] , then the target will exist only on the left of mid?
Perform recursive binary search over some custom example?
The 2014 “Catalogue of Life” contains about 1580000 names of species. If these names were sorted in an array, in the worst case, how many comparisons would it take for the binary search to find the name of a particular species in the array?
Can you list the places where is Binary search algorithm is used?