src/algorithms/uncategorized/rain-terraces/dpRainTerraces.js

@@ -5,11 +5,43 @@
  * @return {number}
  */
 export default function dpRainTerraces(terraces) {
-  // Obtain the maximum elevations to the left and right of each terrace (inclusive).
-  const maxL = terraces.reduce((acc, val, idx) => [...acc, Math.max(val, (acc[idx - 1] || 0))], []);
-  const maxR = terraces.reduceRight((acc, val) => [Math.max(val, (acc[0] || 0)), ...acc], []);
+  let waterAmount = 0;
 
-  // Convert maximum values to amount of water at current index and sum water values.
-  const trap = terraces.map((val, idx) => Math.max(0, Math.min(maxL[idx], maxR[idx]) - val));
-  return trap.reduce((acc, val) => acc + val);
+  // Init arrays that will keep the list of left and right maximum levels for specific positions.
+  const leftMaxLevels = new Array(terraces.length).fill(0);
+  const rightMaxLevels = new Array(terraces.length).fill(0);
+
+  // Calculate the highest terrace level from the LEFT relative to the current terrace.
+  [leftMaxLevels[0]] = terraces;
+  for (let terraceIndex = 1; terraceIndex < terraces.length; terraceIndex += 1) {
+    leftMaxLevels[terraceIndex] = Math.max(
+      terraces[terraceIndex],
+      leftMaxLevels[terraceIndex - 1],
+    );
+  }
+
+  // Calculate the highest terrace level from the RIGHT relative to the current terrace.
+  rightMaxLevels[terraces.length - 1] = terraces[terraces.length - 1];
+  for (let terraceIndex = terraces.length - 2; terraceIndex >= 0; terraceIndex -= 1) {
+    rightMaxLevels[terraceIndex] = Math.max(
+      terraces[terraceIndex],
+      rightMaxLevels[terraceIndex + 1],
+    );
+  }
+
+  // Not let's go through all terraces one by one and calculate how much water
+  // each terrace may accumulate based on previously calculated values.
+  for (let terraceIndex = 0; terraceIndex < terraces.length; terraceIndex += 1) {
+    // Pick the lowest from the left/right highest terraces.
+    const currentTerraceBoundary = Math.min(
+      leftMaxLevels[terraceIndex],
+      rightMaxLevels[terraceIndex],
+    );
+
+    if (currentTerraceBoundary > terraces[terraceIndex]) {
+      waterAmount += currentTerraceBoundary - terraces[terraceIndex];
+    }
+  }
+
+  return waterAmount;
 }