Add tripleStep, recursion folder

08_recursion_and_dynamic_programming/01_tripleStep.js

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+/* 
+  A child is running up a staircase with n steps and can hop
+  either 1 step, 2 steps, or 3 steps at a time. Implement a method to
+  count how many possible ways the child can run up the stairs.
+*/
+
+function tripleStep(n) {
+  // Your code here
+}
+
+module.exports = tripleStep;

08_recursion_and_dynamic_programming/01_tripleStep.test.js

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+'use strict';
+
+const assert = require('chai').assert;
+const tripleStep = require('./01_tripleStep');
+
+describe('recursion and dynamic programming', function() {
+  describe('triple step', function() {
+    it('should find the number of ways to hop N steps', function() {
+      assert.equal(tripleStep(3), 4);
+    })
+    it('should return 0 for non positive numbers', function() {
+      assert.equal(tripleStep(-2), 0);
+    })
+    it('should return 1 for 0', function() {
+      assert.equal(tripleStep(0), 1);
+    })
+  })
+})