add: 第一个只出现一次的字符

test/array/first-unique-character-in-a-string.test.js

@@ -0,0 +1,5 @@
+import { firstUniqChar } from '../../src/array/first-unique-character-in-a-string'
+
+test('第一个只出现一次的字符', () => {
+  expect(firstUniqChar('leetcode')).toBe('l')
+})