3. 无重复字符的最长子串

/**
 * @param {string} s
 * @return {number}
 */
var lengthOfLongestSubstring = function (s) {
  let result = '',
    max = 0
  for (let i = 0; i < s.length; i++) {
    if (result.indexOf(s[i]) === -1) {
      result += s[i]
      max = Math.max(result.length, max)
    } else {
      result = result.slice(result.indexOf(s[i]) + 1) + s[i]
    }
  }
  return max
}

优化的 Map

/**
 * @param {string} s
 * @return {number}
 */
var lengthOfLongestSubstring = function (s) {
  let map = new Map(),
    max = 0
  for (let i = 0, j = 0; j < s.length; j++) {
    if (map.has(s[j])) {
      i = Math.max(map.get(s[j]) + 1, i)
    }
    max = Math.max(max, j - i + 1)
    map.set(s[j], j)
  }
  return max
}