112. Path Sum

solutions/pathSum.test.ts

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+import { createBinaryTreeNode } from "../utilities/TreeNode";
+import pathSum from "./pathSum";
+
+describe("112. Path Sum", () => {
+  const TEST_CASES = new Map<[(number | null)[], number], boolean>([
+    [[[5, 4, 8, 11, null, 13, 4, 7, 2, null, null, null, 1], 22], true],
+    [[[5, 4, 8, 11, null, 13, 4, 7, 2, null, null, null, 1], 20], false],
+    [[[], 0], false]
+  ]);
+
+  for (const [[values, sum], expected] of TEST_CASES) {
+    it(`returns ${expected} when called with [${values}] and ${sum}`, () => {
+      expect(pathSum(createBinaryTreeNode(values), sum)).toBe(expected);
+    });
+  }
+});

solutions/pathSum.ts

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+import { TreeNode } from "../utilities/TreeNode";
+
+// 112. Path Sum
+// https://leetcode.com/problems/path-sum/
+export default function hasPathSum(
+  root: TreeNode<number> | null,
+  sum: number
+): boolean {
+  // ⚠️ as following the test cases, we need to return false for hasPathSum(null, 0)
+  //    so it cannot be just `if (root === null) return sum === 0`
+  if (root === null) return false;
+  if (root.left === null && root.right === null) return root.val === sum;
+
+  // subtract self value and check the the leaf's value matches with sum
+  return (
+    hasPathSum(root.left, sum - root.val) ||
+    hasPathSum(root.right, sum - root.val)
+  );
+}