js笔试算法题 字符串压缩、字符串解压缩

// 压缩字符串 'aaaaaabbbbbcccca' => 'a6b5c4a1'
function compressStr(strs = 'aaaaaabbbbbccccaa') {
  let pre = strs[0]
  let curNum = 1
  let res = ''
  for (let i = 1; i < strs.length; i++) {
    const str = strs[i]
    if (str === pre) {
      curNum++
    } else {
      res += pre
      res += curNum
      pre = str
      curNum = 1
    }
  }
  res += pre
  res += curNum
  return res.length > strs.length ? strs : res
}
compressStr()

// 输入：s = "3[a]2[bc]"
// 输出："aaabcbc"
const decodeString = (s = "3[a]2[bc]") => {
  let numStack = [] // 存倍数的栈
  let strStack = [] // 存 待拼接的str 的栈
  let num = 0 // 倍数的“搬运工”
  let result = '' // 字符串的“搬运工”
  for (const char of s) {
    // 逐字符扫描
    if (/\d/.test(char)) {
      // 遇到数字
      num = num * 10 + Number(char) // 算出倍数
    } else if (char == '[') {
      // 遇到 [
      strStack.push(result) // result串入栈
      result = '' // 入栈后清零
      numStack.push(num) // 倍数num进入栈等待
      num = 0 // 入栈后清零
    } else if (char == ']') {
      // 遇到 ]，两个栈的栈顶出栈
      let repeatTimes = numStack.pop() // 获取拷贝次数
      result = strStack.pop() + result.repeat(repeatTimes) // 构建子串
    } else {
      result += char // 遇到字母，追加给result串
    }
  }
  return result
}
decodeString()