Add solution #224

Author: JoshCrozier

README.md

@@ -198,6 +198,7 @@
 219|[Contains Duplicate II](./0219-contains-duplicate-ii.js)|Easy|
 220|[Contains Duplicate III](./0220-contains-duplicate-iii.js)|Hard|
 222|[Count Complete Tree Nodes](./0222-count-complete-tree-nodes.js)|Easy|
+224|[Basic Calculator](./0224-basic-calculator.js)|Hard|
 225|[Implement Stack using Queues](./0225-implement-stack-using-queues.js)|Easy|
 226|[Invert Binary Tree](./0226-invert-binary-tree.js)|Easy|
 228|[Summary Ranges](./0228-summary-ranges.js)|Easy|

solutions/0224-basic-calculator.js

@@ -0,0 +1,45 @@
+/**
+ * 224. Basic Calculator
+ * https://leetcode.com/problems/basic-calculator/
+ * Difficulty: Hard
+ *
+ * Given a string s representing a valid expression, implement a basic calculator to
+ * evaluate it, and return the result of the evaluation.
+ *
+ * Note: You are not allowed to use any built-in function which evaluates strings as
+ * mathematical expressions, such as eval().
+ */
+
+/**
+ * @param {string} s
+ * @return {number}
+ */
+var calculate = function(s) {
+  const stack = [];
+  let result = 0;
+
+  for (let i = 0, sign = 1; i < s.length; i += 1) {
+    if (s[i] >= '0' && s[i] <= '9') {
+      let value = 0;
+      while (s[i] >= '0' && s[i] <= '9') {
+        value = (value * 10) + (s[i] - '0');
+        i += 1;
+      }
+      result += value * sign;
+      i -= 1;
+    } else if (s[i] === '+') {
+      sign = 1;
+    } else if (s[i] === '-') {
+      sign = -1;
+    } else if (s[i] === '(') {
+      stack.push(result, sign);
+      result = 0;
+      sign = 1;
+    } else if (s[i] === ')') {
+      result = stack.pop() * result;
+      result += stack.pop();
+    }
+  }
+
+  return result;
+};