feat: Stings Missing Number algorithm

Author: v1ktoryy

Strings/missingNumber.js

@@ -0,0 +1,13 @@
+function missingNumber(arr){
+    var n = arr.length+1, 
+    sum = 0,
+    expectedSum = n* (n+1)/2;
+    
+    for(var i = 0, len = arr.length; i < len; i++){
+      sum += arr[i];
+    }
+    
+    return expectedSum - sum;
+  }
+  
+  missingNumber([5, 2, 6, 1, 3]);