在alternatively sorted的数组中查找元素

这里alternatively sorted的数组是指 奇数位置index上的数升序排列 与 偶数位置index上的数升序排列

如12,2,16,5,18,32,33,38

用2次二分查找就可以达到要求

/*
Given an array which is alternatively sorted. Find an element in it.
e.g. 12,2,16,5,18,32,33,38
*/
#include <algorithm>
#include <iostream>

int binarySearch(int* a, int left, int right, int target)
{
	if(left <= right)
	{
		if(target > a[right] || target < a[left])
			return -1;

		int mid = (left + right) / 2;

		if(mid - left == 1 && right - mid == 1)
			return -1;

		if(a[mid] == target)
			return mid;
		else if(a[mid] > target)
			return binarySearch(a, left, mid - 2, target);
		else
			return binarySearch(a, mid + 2, right, target);
	}
};


int findElemIdx(int* a, int n, int target)
{
	if(n == 0)
		return -1;
	else if(n == 1)
	{
		if(target == a[0])
			return 0;
		else
			return -1;
	}
	else
	{
		int endIdx = n - 1;
		int res;

		int oddIdx, evenIdx;
		if(endIdx % 2 == 1)
		{
			oddIdx = endIdx;
			evenIdx = endIdx - 1;
		}
		else
		{
			evenIdx = endIdx;
			oddIdx = endIdx - 1;
		}

		res = binarySearch(a, 1, oddIdx, target);
		
		if(res == -1)
			res = binarySearch(a, 0, evenIdx, target);

		return res;
	}
};



int main()
{
	int a[] = {12,2,16,5,18,32,33,38};

	int res = findElemIdx(a, 8, 2);
	return 0;
}