algorithm: unique paths (#1211)

* dp problem

* update Directory.md

* suggested changes

Dynamic-Programming/tests/UniquePaths.test.js

@@ -0,0 +1,11 @@
+import { uniquePaths } from '../UniquePaths'
+
+describe('Unique Paths', () => {
+  it('should return 28 when m is 3 and n is 7', () => {
+    expect(uniquePaths(3, 7)).toBe(28)
+  })
+
+  it('should return 48620 when m is 10 and n is 10', () => {
+    expect(uniquePaths(10, 10)).toBe(48620)
+  })
+})