/**

• @param {string} s
• @param {string} t
• @return {string}
  */
  var minWindow = function(s, t) {
  let l = 0
  let r = 0
  const need = new Map()
  // 构建需求字典： {目标字符：匹配次数}
  for ( let item of t ) {
  need.set( item, need.has(item) ? need.get(item) + 1 : 1 )
  }
  let needType = need.size
  // 存放最小字串
  let str = ''
  while ( r < s.length ) {
  const ele = s[r]
  // 截取包含目标字符串的子串
  if ( need.has(ele) ) {
  need.set( ele, need.get(ele) - 1 )
  if ( need.get(ele) === 0 ) needType -= 1
  }
  // 当needType为0时说明已全部匹配到,开始移动左指针,当遇到目标字符时给需求字典里该字符需求次数+1,然后继续移动一位
  while ( needType = 0 ) {
  const newStr = s.substring(l, r + 1)
  if ( !str || newStr.length < str.length ) str = newStr
  const ele2 = s[l]
  if ( need.has(ele2) ) {
  need.set(ele2, need.get(ele2) + 1)
  if (need.get(ele2) === 1) needType += 1
  }
  l += 1
  }
  r += 1
  }
  return str
  };