540

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