Given an array of integers `nums` which is sorted in ascending order, and an integer `target`, write a function to search `target` in `nums`. If `target` exists, then return its index. Otherwise, return `-1`.

You must write an algorithm with `O(log n)` runtime complexity.

Example 1:

    Input: nums = [-1,0,3,5,9,12], target = 9
    Output: 4
    Explanation: 9 exists in nums and its index is 4

Example 2:

    Input: nums = [-1,0,3,5,9,12], target = 2
    Output: -1
    Explanation: 2 does not exist in nums so return -1

Constraints:

- `1 <= nums.length <= 104`
- `-104 < nums[i], target < 104`
- All the integers in `nums` are unique.
- `nums` is sorted in ascending order.