FirstPositionOfTarget.java

public class FirstPositionOfTarget {
    /**
         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.

         Challenge
         If the count of numbers is bigger than 2^32, can your code work properly?
     */

    public int binarySearch(int[] nums, int target) {
        if (nums == null) return -1;
        int start = 0, end = nums.length - 1;
        while (start < end - 1) {
            int mid = start + (end - start) / 2;
            if (nums[mid] < target) {
                start = mid;
            } else {
                end = mid;
            }
        }

        if (nums[start] == target) return start;
        if (nums[end] == target) return end;
        return -1;
    }
}