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+# [540. Single Element in a Sorted Array](https://leetcode-cn.com/problems/single-element-in-a-sorted-array/)
+
+## 题目
+
+You are given a sorted array consisting of only integers where every element appears exactly twice, except for one element which appears exactly once.
+
+Return the single element that appears only once.
+
+Your solution must run in O(log n) time and O(1) space.
+
+**Example 1:**
+
+    Input: nums = [1,1,2,3,3,4,4,8,8]
+    Output: 2
+
+**Example 2:**
+
+    Input: nums = [3,3,7,7,10,11,11]
+    Output: 10
+
+**Constraints:**
+
+- 1 <= nums.length <= 100000
+- 0 <= nums[i] <= 100000
+
+## 题目大意
+
+给你一个仅由整数组成的有序数组，其中每个元素都会出现两次，唯有一个数只会出现一次。
+
+请你找出并返回只出现一次的那个数。
+
+你设计的解决方案必须满足 O(log n) 时间复杂度和 O(1) 空间复杂度。
+
+## 解题思路
+
+    假设下标idx是单独的数字,idx左边的偶数下标x有nums[x] == nums[x + 1],
+    idx右边的奇数下标y有nums[y] == nums[y + 1],可以根据此特性用二分查找idx对应的值 
+
+## 代码
+
+```go
+package leetcode
+
+func singleNonDuplicate(nums []int) int {
+	left, right := 0, len(nums)-1
+	for left < right {
+		mid := (left + right) / 2
+		if mid%2 == 0 {
+			if nums[mid] == nums[mid+1] {
+				left = mid + 1
+			} else {
+				right = mid
+			}
+		} else {
+			if nums[mid] == nums[mid-1] {
+				left = mid + 1
+			} else {
+				right = mid
+			}
+		}
+	}
+	return nums[left]
+}
+```