Lintcode 14 First Position of Target

Q: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.

A: binary search

 

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