Add solution #306

Author: JoshCrozier

README.md

@@ -230,6 +230,7 @@
 292|[Nim Game](./0292-nim-game.js)|Easy|
 295|[Find Median from Data Stream](./0295-find-median-from-data-stream.js)|Hard|
 303|[Range Sum Query - Immutable](./0303-range-sum-query-immutable.js)|Easy|
+306|[Additive Number](./0306-additive-number.js)|Medium|
 316|[Remove Duplicate Letters](./0316-remove-duplicate-letters.js)|Medium|
 322|[Coin Change](./0322-coin-change.js)|Medium|
 326|[Power of Three](./0326-power-of-three.js)|Easy|

solutions/0306-additive-number.js

@@ -0,0 +1,45 @@
+/**
+ * 306. Additive Number
+ * https://leetcode.com/problems/additive-number/
+ * Difficulty: Medium
+ *
+ * An additive number is a string whose digits can form an additive sequence.
+ *
+ * A valid additive sequence should contain at least three numbers. Except for the first two
+ * numbers, each subsequent number in the sequence must be the sum of the preceding two.
+ *
+ * Given a string containing only digits, return true if it is an additive number or false
+ * otherwise.
+ *
+ * Note: Numbers in the additive sequence cannot have leading zeros, so sequence 1, 2, 03 or
+ * 1, 02, 3 is invalid.
+ */
+
+/**
+ * @param {string} num
+ * @param {Array} group
+ * @param {number} startIndex
+ * @return {boolean}
+ */
+var isAdditiveNumber = function(num, group = [], startIndex = 0) {
+  if (startIndex === num.length && group.length >= 3) {
+    return true;
+  }
+
+  for (let i = startIndex; i < num.length; i++) {
+    if (num[startIndex] === '0' && i !== startIndex) {
+      break;
+    }
+    const n = +num.slice(startIndex, i + 1);
+    if (group[group.length - 1] + group[group.length - 2] !== n && group.length >= 2) {
+      continue;
+    }
+    group.push(n);
+    if (isAdditiveNumber(num, group, i + 1)) {
+      return true;
+    }
+    group.pop();
+  }
+
+  return false;
+};