Add solution #437

Author: JoshCrozier

README.md

@@ -252,6 +252,7 @@
 415|[Add Strings](./0415-add-strings.js)|Easy|
 419|[Battleships in a Board](./0419-battleships-in-a-board.js)|Medium|
 435|[Non-overlapping Intervals](./0435-non-overlapping-intervals.js)|Medium|
+437|[Path Sum III](./0437-path-sum-iii.js)|Medium|
 442|[Find All Duplicates in an Array](./0442-find-all-duplicates-in-an-array.js)|Medium|
 443|[String Compression](./0443-string-compression.js)|Medium|
 448|[Find All Numbers Disappeared in an Array](./0448-find-all-numbers-disappeared-in-an-array.js)|Easy|

solutions/0437-path-sum-iii.js

@@ -0,0 +1,35 @@
+/**
+ * 437. Path Sum III
+ * https://leetcode.com/problems/path-sum-iii/
+ * Difficulty: Medium
+ *
+ * Given the root of a binary tree and an integer targetSum, return the number of paths where the
+ * sum of the values along the path equals targetSum.
+ *
+ * The path does not need to start or end at the root or a leaf, but it must go downwards
+ * (i.e., traveling only from parent nodes to child nodes).
+ */
+
+/**
+ * Definition for a binary tree node.
+ * function TreeNode(val, left, right) {
+ *     this.val = (val===undefined ? 0 : val)
+ *     this.left = (left===undefined ? null : left)
+ *     this.right = (right===undefined ? null : right)
+ * }
+ */
+/**
+ * @param {TreeNode} root
+ * @param {number} sum
+ * @return {number}
+ */
+var pathSum = function(root, sum) {
+  if (!root) return 0;
+  return (subPathSum(root, sum) + pathSum(root.left, sum) + pathSum(root.right, sum));
+};
+
+function subPathSum(node, sum) {
+  if (!node) return 0;
+  const self = node.val === sum ? 1 : 0;
+  return self + subPathSum(node.left, sum - node.val) + subPathSum(node.right, sum - node.val);
+}