@@ -0,0 +1,48 @@
+/**
+ * @param {string} digits
+ * @return {string[]}
+ */
+var letterCombinations = function(digits) {
+  let sanitizeInput = function(unsanitizedDigits) {
+    let regex = /(\W|_|[0-1a-zA-Z])/g;
+    let sanitized = unsanitizedDigits.replace(regex, "");
+    return sanitized;
+  };
+  const mapping = {
+    2: ["a", "b", "c"],
+    3: ["d", "e", "f"],
+    4: ["g", "h", "i"],
+    5: ["j", "k", "l"],
+    6: ["m", "k", "o"],
+    7: ["p", "q", "r", "s"],
+    8: ["t", "u", "v"],
+    9: ["w", "x", "y", "z"],
+  };
+  let sanitizedDigits = sanitizeInput(digits);
+  if (sanitizedDigits.length > 12) {
+    return "1,388,674 combinations should be more than enough for you. Provide 12 or fewer digits.";
+  }
+  if (sanitizedDigits.length < 1) {
+    return "Nothing to return.";
+  }
+  let finalArray = [];
+
+  const backtrackRecursive = function(combination, nextDigits) {
+    if (nextDigits.length === 0) {
+      finalArray.push(combination);
+    } else {
+      for (let i = 0; i < mapping[nextDigits[0]].length; i += 1) {
+        backtrackRecursive(
+          combination + mapping[nextDigits[0]][i],
+          nextDigits.slice(1)
+        );
+      }
+    }
+  };
+
+  if (sanitizedDigits) {
+    backtrackRecursive("", sanitizedDigits);
+  }
+
+  return finalArray;
+};