Add solution #439

Author: JoshCrozier

README.md

@@ -427,6 +427,7 @@
 436|[Find Right Interval](./solutions/0436-find-right-interval.js)|Medium|
 437|[Path Sum III](./solutions/0437-path-sum-iii.js)|Medium|
 438|[Find All Anagrams in a String](./solutions/0438-find-all-anagrams-in-a-string.js)|Medium|
+439|[Ternary Expression Parser](./solutions/0439-ternary-expression-parser.js)|Medium|
 440|[K-th Smallest in Lexicographical Order](./solutions/0440-k-th-smallest-in-lexicographical-order.js)|Hard|
 441|[Arranging Coins](./solutions/0441-arranging-coins.js)|Easy|
 442|[Find All Duplicates in an Array](./solutions/0442-find-all-duplicates-in-an-array.js)|Medium|

solutions/0439-ternary-expression-parser.js

@@ -0,0 +1,38 @@
+/**
+ * 439. Ternary Expression Parser
+ * https://leetcode.com/problems/ternary-expression-parser/
+ * Difficulty: Medium
+ *
+ * Given a string expression representing arbitrarily nested ternary expressions,
+ * evaluate the expression, and return the result of it.
+ *
+ * You can always assume that the given expression is valid and only contains digits,
+ * '?', ':', 'T', and 'F' where 'T' is true and 'F' is false. All the numbers in the
+ * expression are one-digit numbers (i.e., in the range [0, 9]).
+ *
+ * The conditional expressions group right-to-left (as usual in most languages), and
+ * the result of the expression will always evaluate to either a digit, 'T' or 'F'.
+ */
+
+/**
+ * @param {string} expression
+ * @return {string}
+ */
+var parseTernary = function(expression) {
+  const stack = [];
+
+  for (let i = expression.length - 1; i >= 0; i--) {
+    const char = expression[i];
+    if (char !== ':' && char !== '?') {
+      stack.push(char);
+    } else if (char === '?') {
+      const condition = expression[i - 1];
+      const trueValue = stack.pop();
+      const falseValue = stack.pop();
+      stack.push(condition === 'T' ? trueValue : falseValue);
+      i--;
+    }
+  }
+
+  return stack[0];
+};