---
id: 5900f37b1000cf542c50fe8e
challengeType: 5
title: 'Problem 15: Lattice paths'
videoUrl: ''
localeTitle: 问题15：格子路径
---

## Description
<section id="description">从2×2网格的左上角开始，只能向右和向下移动，右下角有6条路线。 <img class="img-responsive center-block" alt="6 2乘2网格的图表显示了右下角的所有路线" src="https://i.imgur.com/1Atixoj.gif"><p>通过给定的<code>gridSize</code>有多少这样的路由？ </p></section>

## Instructions
<section id="instructions">
</section>

## Tests
<section id='tests'>

```yml
tests:
  - text: <code>latticePaths(4)</code>应该返回70。
    testString: 'assert.strictEqual(latticePaths(4), 70, "<code>latticePaths(4)</code> should return 70.");'
  - text: <code>latticePaths(9)</code>应该返回48620。
    testString: 'assert.strictEqual(latticePaths(9), 48620, "<code>latticePaths(9)</code> should return 48620.");'
  - text: <code>latticePaths(20)</code>应该返回137846528820。
    testString: 'assert.strictEqual(latticePaths(20), 137846528820, "<code>latticePaths(20)</code> should return 137846528820.");'

```

</section>

## Challenge Seed
<section id='challengeSeed'>

<div id='js-seed'>

```js
function latticePaths(gridSize) {
  // Good luck!
  return true;
}

latticePaths(4);

```

</div>



</section>

## Solution
<section id='solution'>

```js
// solution required
```
</section>