diff --git a/Sprint-1/fix/median.js b/Sprint-1/fix/median.js
index b22590bc6..734eced5d 100644
--- a/Sprint-1/fix/median.js
+++ b/Sprint-1/fix/median.js
@@ -6,9 +6,26 @@
 // or 'list' has mixed values (the function is expected to sort only numbers).
 
 function calculateMedian(list) {
-  const middleIndex = Math.floor(list.length / 2);
-  const median = list.splice(middleIndex, 1)[0];
-  return median;
+  // validate input
+  if (!Array.isArray(list)) return null;
+
+  // keep only finite numbers (ignore numeric strings and other types)
+  const numbers = list.filter(
+    (x) => typeof x === "number" && Number.isFinite(x)
+  );
+  if (numbers.length === 0) return null;
+
+  // sort numerically (we can sort the filtered array in-place)
+  numbers.sort((a, b) => a - b);
+
+  const n = numbers.length;
+  const mid = Math.floor(n / 2);
+
+  // odd length -> return middle element
+  if (n % 2 === 1) return numbers[mid];
+
+  // even length -> average of two middle values
+  return (numbers[mid - 1] + numbers[mid]) / 2;
 }
 
 module.exports = calculateMedian;
diff --git a/Sprint-1/implement/dedupe.js b/Sprint-1/implement/dedupe.js
index 781e8718a..cdb72a4a4 100644
--- a/Sprint-1/implement/dedupe.js
+++ b/Sprint-1/implement/dedupe.js
@@ -1 +1,20 @@
-function dedupe() {}
+// Remove duplicate primitive values from an array while preserving
+// the first occurrence order. Does not mutate the input array.
+function dedupe(list) {
+  // Defensive: if input is not an array, return an empty array
+  if (!Array.isArray(list)) return [];
+
+  const seen = new Set();
+  const out = [];
+
+  for (const item of list) {
+    if (!seen.has(item)) {
+      seen.add(item);
+      out.push(item);
+    }
+  }
+
+  return out;
+}
+
+module.exports = dedupe;
diff --git a/Sprint-1/implement/dedupe.test.js b/Sprint-1/implement/dedupe.test.js
index d7c8e3d8e..c5859c9d4 100644
--- a/Sprint-1/implement/dedupe.test.js
+++ b/Sprint-1/implement/dedupe.test.js
@@ -16,7 +16,28 @@ E.g. dedupe([1, 2, 1]) returns [1, 2]
 // Given an empty array
 // When passed to the dedupe function
 // Then it should return an empty array
-test.todo("given an empty array, it returns an empty array");
+test("given an empty array, it returns an empty array", () => {
+  expect(dedupe([])).toEqual([]);
+});
+
+test("array with no duplicates returns a copy", () => {
+  const arr = [1, 2, 3];
+  const out = dedupe(arr);
+  expect(out).toEqual(arr);
+  expect(out).not.toBe(arr); // should be a new array
+});
+
+test("removes duplicates while preserving first occurrence order", () => {
+  const input = [5, 1, 1, 2, 3, 2, 5, 8];
+  expect(dedupe(input)).toEqual([5, 1, 2, 3, 8]);
+});
+
+test("does not mutate the original array", () => {
+  const arr = ["a", "a", "b"];
+  const copy = arr.slice();
+  dedupe(arr);
+  expect(arr).toEqual(copy);
+});
 
 // Given an array with no duplicates
 // When passed to the dedupe function
@@ -24,5 +45,5 @@ test.todo("given an empty array, it returns an empty array");
 
 // Given an array of strings or numbers
 // When passed to the dedupe function
-// Then it should return a new array with duplicates removed while preserving the 
+// Then it should return a new array with duplicates removed while preserving the
 // first occurrence of each element from the original array.
diff --git a/Sprint-1/implement/max.js b/Sprint-1/implement/max.js
index 6dd76378e..be0ef52b3 100644
--- a/Sprint-1/implement/max.js
+++ b/Sprint-1/implement/max.js
@@ -1,4 +1,18 @@
 function findMax(elements) {
+  if (!Array.isArray(elements)) return -Infinity;
+
+  // Keep only finite numbers
+  const numbers = elements.filter(
+    (x) => typeof x === "number" && Number.isFinite(x)
+  );
+  if (numbers.length === 0) return -Infinity;
+
+  // Find max without mutating the input
+  let max = -Infinity;
+  for (const n of numbers) {
+    if (n > max) max = n;
+  }
+  return max;
 }
 
 module.exports = findMax;
diff --git a/Sprint-1/implement/max.test.js b/Sprint-1/implement/max.test.js
index 82f18fd88..a35c61e1c 100644
--- a/Sprint-1/implement/max.test.js
+++ b/Sprint-1/implement/max.test.js
@@ -16,28 +16,47 @@ const findMax = require("./max.js");
 // When passed to the max function
 // Then it should return -Infinity
 // Delete this test.todo and replace it with a test.
-test.todo("given an empty array, returns -Infinity");
+test("given an empty array, returns -Infinity", () => {
+  expect(findMax([])).toBe(-Infinity);
+});
 
 // Given an array with one number
 // When passed to the max function
 // Then it should return that number
+test("given an array with one number, returns that number", () => {
+  expect(findMax([42])).toBe(42);
+});
 
 // Given an array with both positive and negative numbers
 // When passed to the max function
 // Then it should return the largest number overall
+test("given an array with both positive and negative numbers, returns the largest number", () => {
+  expect(findMax([-10, 0, 5, 3])).toBe(5);
+});
 
 // Given an array with just negative numbers
 // When passed to the max function
 // Then it should return the closest one to zero
+test("given an array with just negative numbers, returns the closest one to zero", () => {
+  expect(findMax([-10, -5, -3])).toBe(-3);
+});
 
 // Given an array with decimal numbers
 // When passed to the max function
 // Then it should return the largest decimal number
-
+test("given an array with decimal numbers, returns the largest decimal number", () => {
+  expect(findMax([1.5, 2.3, 0.7])).toBe(2.3);
+});
 // Given an array with non-number values
 // When passed to the max function
 // Then it should return the max and ignore non-numeric values
+test("given an array with non-number values, returns the max and ignores non-numeric values", () => {
+  expect(findMax(["hey", 10, "hi", 60, 10])).toBe(60);
+});
 
 // Given an array with only non-number values
 // When passed to the max function
 // Then it should return the least surprising value given how it behaves for all other inputs
+test("given an array with only non-number values, returns -Infinity", () => {
+  expect(findMax(["hey", "hi", "there"])).toBe(-Infinity);
+});
diff --git a/Sprint-1/implement/sum.js b/Sprint-1/implement/sum.js
index 9062aafe3..95a9c89d1 100644
--- a/Sprint-1/implement/sum.js
+++ b/Sprint-1/implement/sum.js
@@ -1,4 +1,13 @@
 function sum(elements) {
+  if (!Array.isArray(elements)) return 0;
+
+  let total = 0;
+  for (const el of elements) {
+    if (typeof el === "number") {
+      total += el;
+    }
+  }
+  return total;
 }
 
 module.exports = sum;
diff --git a/Sprint-1/implement/sum.test.js b/Sprint-1/implement/sum.test.js
index dd0a090ca..91b86f3bb 100644
--- a/Sprint-1/implement/sum.test.js
+++ b/Sprint-1/implement/sum.test.js
@@ -13,24 +13,46 @@ const sum = require("./sum.js");
 // Given an empty array
 // When passed to the sum function
 // Then it should return 0
-test.todo("given an empty array, returns 0")
+test("given an empty array, returns 0", () => {
+  expect(sum([])).toBe(0);
+});
 
 // Given an array with just one number
 // When passed to the sum function
 // Then it should return that number
+test("given an array with one number, returns that number", () => {
+  expect(sum([42])).toBe(42);
+});
 
+// Given an array with multiple numbers
+// When passed to the sum function
+// Then it should return the total sum of those numbers
+test("given an array with multiple numbers, returns the correct sum", () => {
+  expect(sum([10, 20, 30])).toBe(60);
+});
 // Given an array containing negative numbers
 // When passed to the sum function
 // Then it should still return the correct total sum
-
+test("given an array with negative numbers, returns the correct sum", () => {
+  expect(sum([-10, 20, -5])).toBe(5);
+});
 // Given an array with decimal/float numbers
 // When passed to the sum function
 // Then it should return the correct total sum
+test("given an array with decimal numbers, returns the correct sum", () => {
+  expect(sum([1.5, 2.5, 3.5])).toBe(7.5);
+});
 
 // Given an array containing non-number values
 // When passed to the sum function
 // Then it should ignore the non-numerical values and return the sum of the numerical elements
+test("given an array containing non-number values, returns the correct sum", () => {
+  expect(sum(["hey", 10, "hi", 60, 10])).toBe(80);
+});
 
 // Given an array with only non-number values
 // When passed to the sum function
 // Then it should return the least surprising value given how it behaves for all other inputs
+test("given an array with only non-number values, returns 0", () => {
+  expect(sum(["hey", "hi", "there"])).toBe(0);
+});
diff --git a/Sprint-1/refactor/includes.js b/Sprint-1/refactor/includes.js
index 29dad81f0..8c9ae2e66 100644
--- a/Sprint-1/refactor/includes.js
+++ b/Sprint-1/refactor/includes.js
@@ -1,8 +1,7 @@
 // Refactor the implementation of includes to use a for...of loop
 
 function includes(list, target) {
-  for (let index = 0; index < list.length; index++) {
-    const element = list[index];
+  for (const element of list) {
     if (element === target) {
       return true;
     }
diff --git a/Sprint-1/stretch/aoc-2018-day1/solution.js b/Sprint-1/stretch/aoc-2018-day1/solution.js
index e69de29bb..9263787c8 100644
--- a/Sprint-1/stretch/aoc-2018-day1/solution.js
+++ b/Sprint-1/stretch/aoc-2018-day1/solution.js
@@ -0,0 +1,32 @@
+const fs = require("fs");
+const path = require("path");
+
+// Read input relative to this file so script can be run from anywhere
+const data = fs.readFileSync(path.join(__dirname, "input.txt"), "utf8");
+const changes = data
+  .split(/\r?\n/)
+  .map((s) => s.trim())
+  .filter(Boolean)
+  .map(Number);
+
+// Part 1: final frequency after one pass
+const finalFrequency = changes.reduce((acc, change) => acc + change, 0);
+
+// Part 2: first repeated cumulative frequency (iterate the list repeatedly)
+function findFirstDuplicate(arr) {
+  const seen = new Set([0]);
+  let freq = 0;
+
+  while (true) {
+    for (const change of arr) {
+      freq += change;
+      if (seen.has(freq)) return freq;
+      seen.add(freq);
+    }
+  }
+}
+
+const firstDuplicate = findFirstDuplicate(changes);
+
+console.log("Final frequency (part 1):", finalFrequency);
+console.log("First repeated frequency (part 2):", firstDuplicate);