Add solution #679

Author: JoshCrozier

README.md

@@ -512,6 +512,7 @@
 676|[Implement Magic Dictionary](./0676-implement-magic-dictionary.js)|Medium|
 677|[Map Sum Pairs](./0677-map-sum-pairs.js)|Medium|
 678|[Valid Parenthesis String](./0678-valid-parenthesis-string.js)|Medium|
+679|[24 Game](./0679-24-game.js)|Hard|
 680|[Valid Palindrome II](./0680-valid-palindrome-ii.js)|Easy|
 684|[Redundant Connection](./0684-redundant-connection.js)|Medium|
 686|[Repeated String Match](./0686-repeated-string-match.js)|Easy|

solutions/0679-24-game.js

@@ -0,0 +1,48 @@
+/**
+ * 679. 24 Game
+ * https://leetcode.com/problems/24-game/
+ * Difficulty: Hard
+ *
+ * You are given an integer array cards of length 4. You have four cards, each containing a number
+ * in the range [1, 9]. You should arrange the numbers on these cards in a mathematical expression
+ * using the operators ['+', '-', '*', '/'] and the parentheses '(' and ')' to get the value 24.
+ *
+ * You are restricted with the following rules:
+ * - The division operator '/' represents real division, not integer division.
+ *   - For example, 4 / (1 - 2 / 3) = 4 / (1 / 3) = 12.
+ * - Every operation done is between two numbers. In particular, we cannot use '-' as a
+ *   unary operator.
+ *   - For example, if cards = [1, 1, 1, 1], the expression "-1 - 1 - 1 - 1" is not allowed.
+ * - You cannot concatenate numbers together
+ *   - For example, if cards = [1, 2, 1, 2], the expression "12 + 12" is not valid.
+ *
+ * Return true if you can get such expression that evaluates to 24, and false otherwise.
+ */
+
+/**
+ * @param {number[]} cards
+ * @return {boolean}
+ */
+const judgePoint24 = function(cards) {
+  if (cards.length === 1) return Math.abs(cards[0] - 24) < 0.1;
+
+  for (let i = 0; i < cards.length; i++) {
+    for (let j = i + 1; j < cards.length; j++) {
+      const remaining = new Array(cards.length - 1);
+      for (let index = 0, current = 0; current < cards.length; current++) {
+        if (i === current || j === current) continue;
+        remaining[index++] = cards[current];
+      }
+      const a = cards[i];
+      const b = cards[j];
+      const operations = [a + b, a - b, b - a, a * b, a / b, b / a];
+      for (const result of operations) {
+        if (result === 0) continue;
+        remaining[cards.length - 2] = result;
+        if (judgePoint24(remaining)) return true;
+      }
+    }
+  }
+
+  return false;
+};