main.js

/**
 * @param {string} s
 * @param {string} t
 * @return {string}
 */
 var minWindow = function(s, t) {
  //初始化双指针，维护滑动窗口
  let l = 0;
  let r = 0;

  //字典： 滑动窗口需要的字符 和 它的个数
  const need = new Map();
  for(let c of t){
      need.set(c,need.has(c)?need.get(c)+1:1)
  }
  //eg 对于"ABC"的输入，need是 Map{'A'=>1,'B'=>1,'C'=>1}
  let needType = need.size;
  let res = '';
  //移动右指针
  while(r<s.length){
      //获取当前字符
      const c = s[r];
      //如果字符在需要的字符列表里
      if(need.has(c)){
          need.set(c,need.get(c)-1);
          if(need.get(c) === 0) needType -=1;
      }
      //如果找到包含t的子串，就移动左指针
      while(needType === 0){
          //找到包含t的子串其中最小的
          const newRes = s.substring(l,r+1);
          if(!res || newRes.length < res.length) res = newRes;
          const c2 = s[l];
          if(need.has(c2)){
              need.set(c2,need.get(c2)+1);
              if(need.get(c2) === 1) needType+=1;
          }
          l+=1;
      }
      r+=1;
  }
  return res;
};