First Position of Target

解题小规律：

当 mid < target, 小start，大end first end， last start；

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