Array technique 1: prefix and suffix subarrays

Author: ChauDinh

Data Structures/Array/index.js

@@ -0,0 +1,66 @@
+/**
+ * Get products of all other elements
+ *
+ * Given an array of integers, return new array such that each element at index i of
+ * the new array is the product of all the numbers in the original array except the one at i.
+ */
+
+function products(input) {
+  if (input.length === 0) return [];
+
+  let prod = 1;
+  for (let i = 0; i < input.length; i++) {
+    prod *= input[i];
+  }
+
+  let results = new Array(input.length).fill(null);
+  for (let i = 0; i < input.length; i++) {
+    results[i] = prod / input[i];
+  }
+
+  return results;
+}
+
+console.log(products([1, 2, 3, 4, 5]));
+
+function productsFollowingUp(input) {
+  if (input.length === 0) return [];
+
+  // Precomputing prefix sum
+  let prefix = new Array(input.length).fill(null);
+  for (let i = 0; i < input.length; i++) {
+    if (prefix[i - 1]) {
+      prefix[i] = prefix[i - 1] * input[i];
+    } else {
+      prefix[i] = input[i];
+    }
+  }
+
+  // Precomputing suffix sum
+  let suffix = new Array(input.length).fill(null);
+  let reverseInput = input.reverse();
+  for (let i = 0; i < reverseInput.length; i++) {
+    if (suffix[i - 1]) {
+      suffix[i] = suffix[i - 1] * reverseInput[i];
+    } else {
+      suffix[i] = reverseInput[i];
+    }
+  }
+  suffix.reverse();
+
+  // Compute the results based on subarray results
+  let results = new Array(input.length).fill(null);
+  for (let i = 0; i < results.length; i++) {
+    if (i === 0) {
+      results[i] = suffix[i + 1];
+    } else if (i === results.length - 1) {
+      results[i] = prefix[i - 1];
+    } else {
+      results[i] = prefix[i - 1] * suffix[i + 1];
+    }
+  }
+
+  return results;
+}
+
+console.log(productsFollowingUp([1, 2, 3, 4, 5]));