function parseHtml(html) {
  if (!html || !validHtml(html))
    return null

  const matchReg = /<(\w+)>|<\/(\w+)>|([^<]+)/g
  const stack = []
  let result = {}

  html.replace(matchReg, (_, startTag, endTag, text) => {
    if (startTag) {
      const node = {
        tagName: startTag,
        children: [],
      }
      if (stack.length > 0)
        stack[stack.length - 1].children.push(node)
      else
        result = node

      stack.push(node)
    }
    else if (endTag) {
      stack.pop()
    }
    else if (text) {
      stack[stack.length - 1].children.push(text)
    }
  })

  return result
}

/**
 * 检验是否合法的html
 */
function validHtml(html) {
  const stack = []
  const reg = /<\/?(\w+)>/g
  let res = ''

  while ((res = reg.exec(html)) !== null) {
    const tag = res[0]
    const text = res[1]
    const isEnd = tag[1] === '/'

    if (!isEnd) {
      stack.push(text)
    }
    else {
      const lastTag = stack.pop()

      if (text !== lastTag)
        return false
    }
  }
  return stack.length === 0
}
const a = parseHtml('<div>aaa<h1>test</h1></div>')

console.log(a)

/** 输出结果
 * {
    tagName: "div",
    children: [
      "aaa",
      {
        tagName: "h1",
        children: [
          "test",
        ],
      },
    ],
  }
 */

// 输入
const input = {
  A: ['B', 'C'],
  B: ['D'],
  C: ['A', 'E'],
  D: ['A', 'C']
}
/**
 * A -> B -> D -> A (*)
 * A -> C -> A (*)
 * A -> C -> E ()
 */

// 输出
const result = [
  'A -> B -> D',
  'A -> C'
]