Add Strings

Author: charlesbuczel

0415_addStrings.js

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+/**
+ * @param {string} num1 First number to be added, represented as string.
+ * @param {string} num2 Second number to be added, represented as string.
+ * @return {string} String representing sum of num1 and num2.
+ * @summary Add Strings {@link https://leetcode.com/problems/add-strings/}
+ * @description Given two numbers represented as strings, return sum of them. No built-in library for handling big numbers allowed.
+ * Space O(n) - Where n is Math.max(num1.length, num2.length).
+ * Time O(n) - Where n is Math.max(num1.length, num2.length).
+ */
+const addStrings = (num1, num2) => {
+  const answer = [];
+
+  let carryOver = 0;
+  let index1 = num1.length - 1;
+  let index2 = num2.length - 1;
+
+  while (index1 >= 0 || index2 >= 0) {
+    const n1 = index1 >= 0 ? +num1[index1] : 0;
+    const n2 = index2 >= 0 ? +num2[index2] : 0;
+
+    const sum = n1 + n2 + carryOver;
+
+    carryOver = sum > 9 ? 1 : 0;
+    answer.unshift(sum % 10);
+
+    index1--;
+    index2--;
+  }
+
+  if (carryOver) answer.unshift(1);
+
+  return answer.join('');
+};