add: leetcode 540 solution

leetcode/0540.Single-Element-in-a-Sorted-Array/540.Single Element in a Sorted Array.go

@@ -0,0 +1,22 @@
+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]
+}