add: 字符串解码

README.md

@@ -369,6 +369,11 @@ LeetCode 与 LintCode 解题记录。此为个人练习仓库，代码中对重
   - LeetCode 155. 最小栈 <https://leetcode-cn.com/problems/min-stack/>
   - LintCode 12. 带最小值操作的栈 <https://www.lintcode.com/problem/min-stack/description>
 
+- [字符串解码](src/stack/decode-string.js)
+
+  - LeetCode 394. 字符串解码 <https://leetcode-cn.com/problems/decode-string/>
+  - LintCode 575. 字符串解码 <https://www.lintcode.com/problem/decode-string/description>
+
 ## 数学
 
 - [阶乘后的零](src/math/factorial-trailing-zeroes.js)

src/stack/decode-string.js

@@ -0,0 +1,36 @@
+// s = "3[a]2[bc]", 返回 "aaabcbc".
+// s = "3[a2[c]]", 返回 "accaccacc".
+// s = "2[abc]3[cd]ef", 返回 "abcabccdcdcdef".
+/**
+ * @param {string} s
+ * @return {string}
+ */
+export const decodeString = function (s) {
+  const stack = []
+  let multiple = ''
+  for (let i = 0, len = s.length; i < len; i++) {
+    if (!isNaN(Number(s[i]))) { // 判断是数字
+      if (i === 0 || !isNaN(Number(s[i - 1]))) { // 如果上一位也是数字，则
+        multiple += s[i]
+      } else {
+        multiple = s[i]
+      }
+    } else if (multiple && s[i] === '[') {
+      stack.push(Number(multiple))
+      multiple = ''
+    } else if (s[i] === ']') {
+      let current = stack.pop()
+      let tmpStr = ''
+      while (typeof current !== 'number') {
+        tmpStr = current + tmpStr
+        current = stack.pop()
+      }
+      tmpStr = tmpStr.repeat(current)
+      stack.push(tmpStr)
+    } else {
+      stack.push(s[i])
+    }
+  }
+
+  return stack.join('')
+}

test/stack/decode-string.test.js

@@ -0,0 +1,7 @@
+import { decodeString } from '../../src/stack/decode-string'
+
+test('字符串解码', () => {
+  expect(decodeString('3[a]2[bc]')).toBe('aaabcbc')
+  expect(decodeString('3[a2[c]]')).toBe('accaccacc')
+  expect(decodeString('2[abc]3[cd]ef')).toBe('abcabccdcdcdef')
+})