当数组具有奇数值时如何使用二进制搜索？

Question

I'm using binarySearch and it's worked well with arrays that has even index, but when the array has odd index (length) it's give wrong result.

What i'v done so fare:

public static int binarySearch(int[] list, int key) {

    int low = 0;
    int high = list.length - 1;
    while (high >= low) {
        int mid = (low + high) / 2;

        if (key < list[mid])
            high = mid - 1;
        else if (key == list[mid])
            return mid;
        else
            low = mid + 1;

    }
    return - 1;
}

Input:

int[] arr1 = {5, 6, 8, 9, 11, 12, 11, 50, 1, 3, 15, 121, 33, 16, 17, 18, 19};
int[] arr2 = {5, 6, 8, 9, 11, 12, 11, 50, 1, 3, 15, 121, 33, 16, 17, 18};

Case:

System.out.println(binarySearch(arr1, 12));
System.out.println(binarySearch(arr2, 12));

OutPut:

-1
5

How i can get the right outPut in the both situation?

Answer 1

Binary search only works on sorted array

Solution : add Arrays.sort(list)

public static int binarySearch(int[] list, int key) { 
    Arrays.sort(list);
    int low = 0;
    int high = list.length - 1;
    while (high >= low) {
        int mid = (low + high) / 2;
        if (key < list[mid]) high = mid - 1;
        else if (key == list[mid]) return mid;
        else low = mid + 1;
    } return - 1;
}

Answer 2

You must sort the arrays before doing binary search operation.

In your question you are unable to search for the odd array length.

In java its very easy.

You can use the below code.

 import java.util.Arrays; class BS { public static void main(String args[]) { int[] arr1 = {5, 6, 8, 9, 11, 12, 11, 50, 1, 3, 15, 121, 33, 16, 17, 18, 19}; System.out.println(Arrays.binarySearch(arr1 , '12')); } }