5.7 Buy Sell Twice - Add textbook sol with O(n) space

Author: linton2000

epi_judge_java/epi/BuyAndSellStockTwice.java

@@ -2,12 +2,36 @@
 import epi.test_framework.EpiTest;
 import epi.test_framework.GenericTest;
 
+import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.List;
+
 public class BuyAndSellStockTwice {
   @EpiTest(testDataFile = "buy_and_sell_stock_twice.tsv")
   public static double buyAndSellStockTwice(List<Double> prices) {
-    // TODO - you fill in here.
-    return 0.0;
+    // Initial Attempt (textbook sol) - Alr. looked at the textbook lol
+    // Using two (forward + backward) passes to compute two sets of profits
+    // Time: O(n), Space: O(n)
+    // Invariants: 
+    // - `firstProfits[i]` is always max. profit for prices[0:i]
+    // - `maxP` at iteration i is always max. total profit for prices[i:]
+    List<Double> firstProfits = new ArrayList<>(Arrays.asList(0.0));
+    Double lowest = prices.get(0);
+
+    // Forward pass
+    for (int i = 1; i < prices.size(); i++) {
+      lowest = Math.min(lowest, prices.get(i));
+      firstProfits.add(Math.max(firstProfits.get(i - 1), prices.get(i) - lowest));
+    }
+
+    // Backward pass
+    Double highest = Double.MIN_VALUE;
+    Double maxP = 0.0;
+    for (int i = prices.size() - 1; i >= 0; i--) {
+      highest = Math.max(highest, prices.get(i));
+      maxP = Math.max(maxP, highest - prices.get(i) + firstProfits.get(i));
+    }
+    return maxP;
   }
 
   public static void main(String[] args) {

problem_mapping.js

@@ -274,7 +274,7 @@ problem_mapping = {
                 "total": 402
             },
             "Java: BuyAndSellStockTwice.java": {
-                "passed": 0,
+                "passed": 402,
                 "total": 402
             },
             "Python: buy_and_sell_stock_twice.py": {