Add dynamic programming version.

Author: trekhleb

src/algorithms/uncategorized/best-time-to-buy-sell-stocks/__tests__/dpBestTimeToBuySellStocks.test.js

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+import dpBestTimeToBuySellStocks from '../dpBestTimeToBuySellStocks';
+
+describe('dpBestTimeToBuySellStocks', () => {
+  it('should find the best time to buy and sell stocks', () => {
+    let visit;
+
+    expect(dpBestTimeToBuySellStocks([1, 5])).toEqual(4);
+
+    visit = jest.fn();
+    expect(dpBestTimeToBuySellStocks([1], visit)).toEqual(0);
+    expect(visit).toHaveBeenCalledTimes(1);
+
+    visit = jest.fn();
+    expect(dpBestTimeToBuySellStocks([1, 5], visit)).toEqual(4);
+    expect(visit).toHaveBeenCalledTimes(2);
+
+    visit = jest.fn();
+    expect(dpBestTimeToBuySellStocks([5, 1], visit)).toEqual(0);
+    expect(visit).toHaveBeenCalledTimes(2);
+
+    visit = jest.fn();
+    expect(dpBestTimeToBuySellStocks([1, 5, 10], visit)).toEqual(9);
+    expect(visit).toHaveBeenCalledTimes(3);
+
+    visit = jest.fn();
+    expect(dpBestTimeToBuySellStocks([10, 1, 5, 20, 15, 21], visit)).toEqual(25);
+    expect(visit).toHaveBeenCalledTimes(6);
+
+    visit = jest.fn();
+    expect(dpBestTimeToBuySellStocks([7, 1, 5, 3, 6, 4], visit)).toEqual(7);
+    expect(visit).toHaveBeenCalledTimes(6);
+
+    visit = jest.fn();
+    expect(dpBestTimeToBuySellStocks([1, 2, 3, 4, 5], visit)).toEqual(4);
+    expect(visit).toHaveBeenCalledTimes(5);
+
+    visit = jest.fn();
+    expect(dpBestTimeToBuySellStocks([7, 6, 4, 3, 1], visit)).toEqual(0);
+    expect(visit).toHaveBeenCalledTimes(5);
+
+    visit = jest.fn();
+    expect(dpBestTimeToBuySellStocks(
+      [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20],
+      visit,
+    )).toEqual(19);
+    expect(visit).toHaveBeenCalledTimes(20);
+  });
+});

src/algorithms/uncategorized/best-time-to-buy-sell-stocks/dpBestTimeToBuySellStocks.js

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+/**
+ * Finds the maximum profit from selling and buying the stocks.
+ * DYNAMIC PROGRAMMING APPROACH.
+ *
+ * @param {number[]} prices - Array of stock prices, i.e. [7, 6, 4, 3, 1]
+ * @param {function(): void} visit - Visiting callback to calculate the number of iterations.
+ * @return {number} - The maximum profit
+ */
+const dpBestTimeToBuySellStocks = (prices, visit = () => {}) => {
+  visit();
+  let lastBuy = -prices[0];
+  let lastSold = 0;
+
+  for (let day = 1; day < prices.length; day += 1) {
+    visit();
+    const curBuy = Math.max(lastBuy, lastSold - prices[day]);
+    const curSold = Math.max(lastSold, lastBuy + prices[day]);
+    lastBuy = curBuy;
+    lastSold = curSold;
+  }
+
+  return lastSold;
+};
+
+export default dpBestTimeToBuySellStocks;