> For the complete documentation index, see [llms.txt](https://liuxue2010.gitbook.io/data-structure-and-algorithms/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://liuxue2010.gitbook.io/data-structure-and-algorithms/binary-search/first-position-of-target.md).

# First Position of Target

For a given sorted array (ascending order) and a target number, find the first index of this number in O(log n) time complexity.

If the target number does not exist in the array, return -1.

## Example

If the array is \[1, 2, 3, 3, 4, 5, 10], for given target 3, return 2.

## Solution

```
class Solution:
    # @param nums: The integer array
    # @param target: Target number to find
    # @return the first position of target in nums, position start from 0 
    def binarySearch(self, nums, target):
        if len(nums) == 0:
            return -1
        start, end = 0, len(nums) -1 
        while start + 1 < end:
            mid = (start + end) / 2
            if nums[mid] < target:
                start = mid
            else:
                end = mid
        if nums[start] == target:
            return start
        if nums[end] == target:
            return end
        return -1
```