题解 | #包围#

function bind() {
  window.addEventListener('click', (e) => {
    if (e.target.tagName !== 'TD') return
    handler(e.target)
  })
  function findIndex(tag, nn){
    let n = -1;
    let out = tag.parentNode
    for(let i=0, imax=out.children.length; i<imax; i++) {
      if (out.children[i] == tag) {
        n = i
      }
    }
    return n
  }
  function findEle(tag, n) {
    let out = tag.parentNode
    let pos = findIndex(tag)
    return out.children[pos+n]
  }
  function renderEle(tag, isMain, n) {
    if (!tag) return
    isMain ? tag.className = 'current' : tag.className = 'wrap';
    let prev = tag.parentNode.children[n-1]
    let next = tag.parentNode.children[n+1]
    next? next.className = 'wrap' : false;
    prev? prev.className = 'wrap' : false;
  }
  function handler(tag){
    let ids = document.getElementsByTagName('td')
    for(let i=0, imax=ids.length; i<imax; i++) {
      ids[i].className = ''
    }
    n = findIndex(tag, 0)

    upTag = findEle(tag.parentNode, -1);
    downTag = findEle(tag.parentNode,1)

    upTag ? renderEle(upTag.children[n], false, n): false
    downTag ? renderEle(downTag.children[n], false, n): false
    renderEle(tag, true, n)
  }
}