res.js

/**
 * 超时 - 分治方法
 * @param {string} s
 * @param {number} k
 * @return {number}
 */
var longestSubstring = function(s, k) {
  const len = s.length;
  if (len<k) return 0;

  const count = (start, end) => {
    if (end-start+1 < k) return 0;

    const strdict = {};
    let numOfLessThanK = 0;
    for (let i = start; i <= end; i++) {
      const e = s[i];
      if (strdict[e] === undefined) {
        strdict[e] = 1;
        numOfLessThanK += 1;
      } else {
        strdict[e] += 1;
        if (strdict[e] === k) {
          numOfLessThanK -= 1;
        }
      };
    }

    if (!numOfLessThanK) return end-start+1;
    while(end-start+1 < k && strdict[start] < k) {
      start++;
    }
    while(end-start+1 < k && strdict[end] < k) {
      end--;
    }
    if (end-start+1 < k) return 0;

    for (let i = start; i <= end; i++) {
      const e = s[i];
      if (strdict[e] < k) {
        let next = i+1;
        while(strdict[next] < k && next < end) next++;
        return Math.max(count(start, i-1),count(next, end));
      }
    }

    return end-start+1;
  }

  return count(0, len-1);
};