[Math] Refactor solution to Add Binary

Math/AddBinary.swift

@@ -1,6 +1,6 @@
 /**
  * Question Link: https://leetcode.com/problems/add-binary/
- * Primary idea: use Carry and iterate from last to start
+ * Primary idea: Two pointers: use carry and iterate from last to start
  * 
  * Note: Swift does not have a way to access a character in a string with O(1), 
  *       thus we have to first transfer the string to a character array
@@ -10,25 +10,25 @@
 
 class AddBinary {
     func addBinary(_ a: String, _ b: String) -> String {
-        var sum = 0, carry = 0, res = ""
-        let aChars = Array(a.characters), bChars = Array(b.characters)
-        var i = aChars.count - 1, j = bChars.count - 1
-
+        let a = Array(a), b = Array(b)
+        var res = "", carry = 0, i = a.count - 1, j = b.count - 1
+
         while i >= 0 || j >= 0 || carry > 0 {
-            sum = carry
+            var sum = carry
+
             if i >= 0 {
-                sum += Int(String(aChars[i]))!
+                sum += Int(String(a[i]))!
                 i -= 1
             }
             if j >= 0 {
-                sum += Int(String(bChars[j]))!
+                sum += Int(String(b[j]))!
                 j -= 1
             }
+
+            res = "\(sum % 2)" + res
             carry = sum / 2
-            sum = sum % 2
-            res = String(sum) + res
         }
-        
+
         return res
     }
 }