adding MissingNumber.dart

Author: Farial-mahmod

MissingNumber.dart

@@ -0,0 +1,20 @@
+// Leetcode problem link: https://leetcode.com/problems/missing-number/
+
+class Solution {
+
+// initializing an iterator at first to avoid initializing it each time inside the loop as the loop iterates
+var i = 0;
+
+// this function will be called with a list to return the missing number
+int missingNumber(List nums) {
+for (; i <= nums.length; i++) {
+    // checking if the list doesn't contain i
+    if (!nums.contains(i)) {
+        return i;
+    }
+}
+
+return 0;
+
+}
+}