create 0136-single-number.dart

Author: bansalgaurav852

Diff for: dart/0136-single-number.dart

@@ -0,0 +1,19 @@
+class Solution {
+  int singleNumber(List<int> nums) {
+    int? val = null;
+    nums.sort();
+    int i = 0;
+    while (i < nums.length - 2) {
+      if (!(nums[i] != nums[i + 2] && nums[i] == nums[i + 1])) {
+        val = nums[i];
+        break;
+      } else {
+        i = i + 2;
+      }
+    }
+    if (val == null) {
+      val = nums[nums.length - 1];
+    }
+    return val;
+  }
+}