[String] Add Solution to Ransom Note

String/RansomNote.swift

@@ -0,0 +1,39 @@
+/**
+ * Question Link: https://leetcode.com/problems/ransom-note/
+ * Primary idea: Use a dictionary to calculate the existence of characters in magazine 
+ *               and check with the ransom Note
+ *
+ * Time Complexity: O(n), Space Complexity: O(n)
+ */
+
+class RansomNote {
+    func canConstruct(ransomNote: String, _ magazine: String) -> Bool {
+        var magazineMap = _strToMap(magazine)
+
+        for char in ransomNote.characters {
+            if magazineMap[char] == nil {
+                return false
+            } else if magazineMap[char] == 0 {
+                return false
+            } else {
+                magazineMap[char]! -= 1 
+            }
+        }
+
+        return true
+    }
+
+    private func _strToMap(magazine: String) -> [Character: Int] {
+        var res = [Character: Int]()
+
+        for char in magazine.characters {
+            if res[char] == nil {
+                res[char] = 1
+            } else {
+                res[char]! += 1
+            }
+        }
+
+        return res
+    }
+}