From 87296e01b8671ac75b2ab3d653f3f92bf5eb474b Mon Sep 17 00:00:00 2001
From: Alex Small <alex.small739@gmail.com>
Date: Wed, 25 Mar 2026 13:53:57 +0000
Subject: [PATCH] Add best time to buy and sell stock solution and test files

---
 .../best-time-to-buy-and-sell-stock/.test.ts  | 81 +++++++++++++++++++
 .../solution.ts                               | 46 +++++++++++
 2 files changed, 127 insertions(+)
 create mode 100644 problems/two-pointers-and-sliding-window/best-time-to-buy-and-sell-stock/.test.ts
 create mode 100644 problems/two-pointers-and-sliding-window/best-time-to-buy-and-sell-stock/solution.ts

diff --git a/problems/two-pointers-and-sliding-window/best-time-to-buy-and-sell-stock/.test.ts b/problems/two-pointers-and-sliding-window/best-time-to-buy-and-sell-stock/.test.ts
new file mode 100644
index 0000000..dfea59e
--- /dev/null
+++ b/problems/two-pointers-and-sliding-window/best-time-to-buy-and-sell-stock/.test.ts
@@ -0,0 +1,81 @@
+import { test, expect, describe } from 'vitest'
+import { nestedLoops, runningMin } from './solution'
+
+const expectCorrectProfit = (inputs: [number[], number][]) => {
+    inputs.forEach(([prices, expected]) => {
+        expect(nestedLoops(prices)).toBe(expected)
+        expect(runningMin(prices)).toBe(expected)
+    })
+}
+
+describe('Positive profit is returned correctly by both solutions when:', () => {
+    test('Best day to buy is minimum price and best day to sell is maximum price.', () => {
+        const inputs = [
+            [[7,1,5,3,6,4], 5],
+            [[1,2,3,4,5], 4],
+            [[5,4,7,10, 11], 7],
+            [[8,4,5,6,20], 16]
+        ] satisfies [number[], number][]
+        expectCorrectProfit(inputs)
+    })
+    test('Maximum price is at the beginning of the array.', () => {
+        const inputs = [
+            [[8,1,2,4], 3],
+            [[10,1,2,3], 2],
+            [[20, 1, 2, 19], 18],
+            [[100, 1, 2, 99], 98]
+        ] satisfies [number[], number][]
+        expectCorrectProfit(inputs)
+    })
+    test('Minimum price is at the end of the array.', () => {
+        const inputs = [
+            [[3,4,6,8, 1], 5],
+            [[7,8,9,2], 2],
+            [[10, 11, 12, 1], 2],
+            [[50, 51, 52, 1], 2]
+        ] satisfies [number[], number][]
+        expectCorrectProfit(inputs)
+    })
+    test('Best day to buy is not minimum price.', () => {
+        const inputs = [
+            [[2,7,1,4], 5],
+            [[3,10,2,3], 7],
+            [[3,20,1,17], 17],
+        ] satisfies [number[], number][]
+        expectCorrectProfit(inputs)
+    })
+})
+
+describe('Both solutions return zero profit correctly when:', () => {
+    test('Prices are in descending order.', () => {
+        const inputs = [
+            [[7,6,4,3,1], 0],
+            [[9,7,4,2,1], 0],
+            [[50, 3, 2, 2, 1], 0],
+            [[100, 99, 97, 97, 97], 0]
+        ] satisfies [number[], number][]
+        expectCorrectProfit(inputs)
+    })
+    test('All prices are the same.', () => {
+        const inputs = [
+            [[1,1,1,1], 0],
+            [[5,5,5,5], 0],
+            [[100, 100, 100, 100], 0],
+            [[999, 999, 999, 999], 0]
+        ] satisfies [number[], number][]
+        expectCorrectProfit(inputs)
+    })
+    test('Only one price is given.', () => {
+        const inputs = [[[1], 0], [[5], 0], [[100], 0], [[999], 0]] satisfies [number[], number][]
+        expectCorrectProfit(inputs)
+    })
+})
+
+describe('Optimized solution runs for large inputs.', () => {
+    test('Input of 10^5 prices runs within time limit.', () => {
+        // Set array length to maximum allowed by problem constraints
+        const n = 10 ** 5
+        const prices = Array.from({ length: n }, (_, i) => i + 1)
+        expect(runningMin(prices)).toBe(n - 1)  
+    })
+})
\ No newline at end of file
diff --git a/problems/two-pointers-and-sliding-window/best-time-to-buy-and-sell-stock/solution.ts b/problems/two-pointers-and-sliding-window/best-time-to-buy-and-sell-stock/solution.ts
new file mode 100644
index 0000000..80e1aa0
--- /dev/null
+++ b/problems/two-pointers-and-sliding-window/best-time-to-buy-and-sell-stock/solution.ts
@@ -0,0 +1,46 @@
+// Solution 1: Brute force nested loops
+// Time O(n^2), Space O(1)
+// Solution 2: Running minimum
+// Time O(n), Space O(1)
+
+/**
+ * Brute force nested loops
+ * Time O(n^2), Space O(1)
+ */
+export function nestedLoops(prices: number[]): number {
+    let max = 0
+
+    const n = prices.length
+
+    for (let i=0; i<n; i++){
+        for (let j=i+1; j<n; j++){
+            const profit = prices[j] - prices[i]
+            if (profit > max){
+                max = profit
+            }
+        }
+    }    
+
+    return max
+};
+
+/**
+ * Running minimum
+ * Time O(n), Space O(1)
+ */
+export function runningMin(prices: number[]): number {
+    if (prices.length === 0) return 0
+    
+    let minPrice = prices[0]
+    let bestProfit = 0
+    let bestProfitToday = 0
+
+    prices.forEach((price) => {
+        bestProfitToday = price - minPrice
+        minPrice = Math.min(price, minPrice)
+        bestProfit = Math.max(bestProfit, bestProfitToday)
+    })
+
+    return bestProfit
+
+};
\ No newline at end of file