BinarySearch.java

package com.rampatra.searching;

/**
 * Created by IntelliJ IDEA.
 *
 * @author rampatra
 * @version 9/1/15
 */
public class BinarySearch {

    /**
     * Searches an element {@param n} in a sorted array {@param a}
     * and returns its index in O(log n) time. The Index may not
     * correspond to the first occurrence of the element.
     *
     * @param a sorted array to be searched
     * @param n number to be searched in the array
     * @return index of {@param n} or {@code -1} if not present
     */
    private static int binarySearch(int[] a, int n) {
        return binarySearch(a, n, 0, a.length - 1);
    }

    public static int binarySearch(int[] a, int n, int low, int high) {

        if (low <= high) {
            int mid = (low + high) / 2; // to prevent overflow you can instead do: mid = low + (high - low) / 2

            if (n == a[mid]) {
                return mid;
            } else if (n < a[mid]) {
                return binarySearch(a, n, 0, mid - 1);
            } else {
                return binarySearch(a, n, mid + 1, high);
            }
        } else {
            return -1;
        }
    }

    /**
     * Non-recursive version of binary search.
     *
     * @param a sorted array to be searched
     * @param n number to be searched in the array
     * @return index of {@param n} or {@code -1} if not present
     */
    private static int binarySearchNonRecursive(int[] a, int n) {
        int low = 0, high = a.length, mid;
        while (low <= high) {
            mid = (low + high) / 2; // to prevent overflow you can instead do: mid = low + (high - low) / 2
            if (n == a[mid]) {
                return mid;
            } else if (n < a[mid]) {
                high = mid - 1;
            } else {
                low = mid + 1;
            }
        }
        return -1;
    }

    /**
     * Driver for testing.
     *
     * @param a
     */
    public static void main(String[] args) {
        System.out.println(binarySearch(new int[]{0, 2}, 2));
        System.out.println(binarySearch(new int[]{0, 1, 2, 3}, 2));
        System.out.println(binarySearch(new int[]{0, 1, 2, 3}, 3));
        System.out.println(binarySearch(new int[]{0, 2}, 0));
        System.out.println(binarySearch(new int[]{0, 1, 2, 2, 2, 3, 3}, 2)); // doesn't return index of first occurrence
        System.out.println("---------");
        System.out.println(binarySearchNonRecursive(new int[]{0, 2}, 2));
        System.out.println(binarySearchNonRecursive(new int[]{0, 1, 2, 3}, 2));
        System.out.println(binarySearchNonRecursive(new int[]{0, 1, 2, 3}, 3));
        System.out.println(binarySearchNonRecursive(new int[]{0, 2}, 0));
        System.out.println(binarySearchNonRecursive(new int[]{0, 1, 2, 2, 2, 3, 3}, 2));
    }
}