Add solution #494

Author: JoshCrozier

README.md

@@ -395,6 +395,7 @@
 491|[Non-decreasing Subsequences](./0491-non-decreasing-subsequences.js)|Medium|
 492|[Construct the Rectangle](./0492-construct-the-rectangle.js)|Easy|
 493|[Reverse Pairs](./0493-reverse-pairs.js)|Hard|
+494|[Target Sum](./0494-target-sum.js)|Medium|
 496|[Next Greater Element I](./0496-next-greater-element-i.js)|Easy|
 500|[Keyboard Row](./0500-keyboard-row.js)|Easy|
 501|[Find Mode in Binary Search Tree](./0501-find-mode-in-binary-search-tree.js)|Easy|

solutions/0494-target-sum.js

@@ -0,0 +1,39 @@
+/**
+ * 494. Target Sum
+ * https://leetcode.com/problems/target-sum/
+ * Difficulty: Medium
+ *
+ * You are given an integer array nums and an integer target.
+ *
+ * You want to build an expression out of nums by adding one of the symbols '+' and '-' before
+ * each integer in nums and then concatenate all the integers.
+ *
+ * For example, if nums = [2, 1], you can add a '+' before 2 and a '-' before 1 and concatenate
+ * them to build the expression "+2-1".
+ *
+ * Return the number of different expressions that you can build, which evaluates to target.
+ */
+
+/**
+ * @param {number[]} nums
+ * @param {number} target
+ * @return {number}
+ */
+var findTargetSumWays = function(nums, target) {
+  const map = new Map();
+
+  function dp(index, sum) {
+    if (index === nums.length) {
+      return sum === target ? 1 : 0;
+    }
+    const key = `${index},${sum}`;
+    if (map.has(key)) {
+      return map.get(key);
+    }
+    const r = dp(index + 1, sum + nums[index]) + dp(index + 1, sum - nums[index]);
+    map.set(key, r);
+    return r;
+  }
+
+  return dp(0, 0);
+};