二分查找法的实现(Binary Search)

Java

// Java implementation of recursive Binary Search
class BinarySearch {
    // Returns index of x if it is present in arr[l..
    // r], else return -1
    int binarySearch(int arr[], int l, int r, int x)
    {
        if (r >= l) {
            int mid = l + (r - l) / 2;
            // 不用int mid = (l + r) / 2;是为了防止整型溢出

            // If the element is present at the
            // middle itself
            if (arr[mid] == x)
                return mid;

            // If element is smaller than mid, then
            // it can only be present in left subarray
            if (arr[mid] > x)
                return binarySearch(arr, l, mid - 1, x);

            // Else the element can only be present
            // in right subarray
            return binarySearch(arr, mid + 1, r, x);
        }

        // We reach here when element is not present
        // in array
        return -1;
    }

    // Driver method to test above
    public static void main(String args[])
    {
        BinarySearch ob = new BinarySearch();
        int arr[] = { 2, 3, 4, 10, 40 };
        int n = arr.length;
        int x = 10;
        int result = ob.binarySearch(arr, 0, n - 1, x);
        if (result == -1)
            System.out.println("Element not present");
        else
            System.out.println("Element found at index " + result);
    }
}
/* This code is contributed by Rajat Mishra */

python

# Python Program for recursive binary search. 

# Returns index of x in arr if present, else -1 
def binarySearch (arr, l, r, x): 

	# Check base case 
	if r >= l: 

		mid = l + (r - l)/2

		# If element is present at the middle itself 
		if arr[mid] == x: 
			return mid 
		
		# If element is smaller than mid, then it 
		# can only be present in left subarray 
		elif arr[mid] > x: 
			return binarySearch(arr, l, mid-1, x) 

		# Else the element can only be present 
		# in right subarray 
		else: 
			return binarySearch(arr, mid + 1, r, x) 

	else: 
		# Element is not present in the array 
		return -1

# Test array 
arr = [ 2, 3, 4, 10, 40 ] 
x = 10

# Function call 
result = binarySearch(arr, 0, len(arr)-1, x) 

if result != -1: 
	print("Element is present at index % d" % result)
else: 
	print("Element is not present in array")

C++

// C++ program to implement recursive Binary Search 
#include <iostream> 
using namespace std; 

// A recursive binary search function. It returns 
// location of x in given array arr[l..r] is present, 
// otherwise -1 
int binarySearch(int arr[], int l, int r, int x) 
{ 
	if (r >= l) { 
		int mid = l + (r - l) / 2; 

		// If the element is present at the middle 
		// itself 
		if (arr[mid] == x) 
			return mid; 

		// If element is smaller than mid, then 
		// it can only be present in left subarray 
		if (arr[mid] > x) 
			return binarySearch(arr, l, mid - 1, x); 

		// Else the element can only be present 
		// in right subarray 
		return binarySearch(arr, mid + 1, r, x); 
	} 

	// We reach here when element is not 
	// present in array 
	return -1; 
} 

int main(void) 
{ 
	int arr[] = { 2, 3, 4, 10, 40 }; 
	int x = 10; 
	int n = sizeof(arr) / sizeof(arr[0]); 
	int result = binarySearch(arr, 0, n - 1, x); 
	(result == -1) ? cout << "Element is not present in array"
				: cout << "Element is present at index " << result; 
	return 0; 
}