Problem:

Given a sorted array consisting of only integers where every element appears twice except for one element which appears once. Find this single element that appears only once.

Example 1:

Input: [1,1,2,3,3,4,4,8,8]
Output: 2

Example 2:

Input: [3,3,7,7,10,11,11]
Output: 10

Note: Your solution should run in O(log n) time and O(1) space.

Idea:

Binary search.

Time complexity: O(logn)

Space complexity: O(1)

Solution: 

C++

// Author: Huahua
// Runtime: 6 ms
class Solution {
public:
    int singleNonDuplicate(vector<int>& nums) {
        int l = 0;
        int r = nums.size();
        while (l < r) {
            const int m = l + (r - l) / 2;
            // int n = m % 2 == 0 ? m + 1 : m - 1;
            const int n = m ^ 1;
            if (nums[m] == nums[n])
                l = m + 1;
            else
                r = m;
        }
        return nums[l];
    }
};

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