// Time:  O(logn)
// Space: O(1)

class Solution {
public:
    int singleNonDuplicate(vector<int>& nums) {
        int left = 0, right = nums.size() - 1;
        while (left <= right) {
            auto mid = left + (right - left) / 2;
            if (!(mid % 2 == 0 && mid + 1 < nums.size() &&
                  nums[mid] == nums[mid + 1]) &&
                !(mid % 2 == 1 && nums[mid] == nums[mid - 1])) {
                right = mid - 1;
            } else {
                left = mid + 1;
            }
        }
        return nums[left];
    }
};