added task #540 solution

Author: nesterrovv

src/task_540/Solution.java

@@ -0,0 +1,20 @@
+package task_540;
+
+public class Solution {
+
+    public int singleNonDuplicate(int[] nums) {
+        int first = 0;
+        int last = nums.length - 1;
+        while (first < last) {
+            int middle = (first + last) / 2;
+            if((middle % 2 == 0 && nums[middle] == nums[middle + 1])
+                    || (middle % 2 == 1 && nums[middle] == nums[middle - 1])) {
+                first = middle + 1;
+            } else {
+                last = middle;
+            }
+        }
+        return nums[first];
+    }
+
+}