nums should contain every integer from start to start + len(nums) inclusive, in increasing order,
but exactly one of them is missing. Return the missing integer.
Examples
Input: start = 3, nums = [3, 4, 5, 7, 8]
Output: 6
Input: start = 1, nums = [2, 3, 4]
Output: 1
Explanation: the range is 1..4 and 1 is absent.
Input: start = 0, nums = [0, 1, 2]
Output: 3
Explanation: the range is 0..3; the last value is the one missing.
Constraints
0 <= len(nums) <= 10**6-10**9 <= start <= 10**9O(log n)time is required; scanning for the gap is too slow on the largest tests.
Goals
- Use the relationship between index and value to locate a gap
- Handle the missing value being at either end of the range