class Solution:
    def singleNonDuplicate(self, nums: List[int]) -> int:
        N = len(nums)
        b, e = 0, N // 2
        ans = nums[-1]
        while b < e:
            m = (b + e) // 2
            if nums[2 * m] == nums[2 * m + 1]:
                b = m + 1
            else:
                ans = nums[2 * m]
                e = m
        return ans