1392-longest-happy-prefix.js

/**
 * @param {string} s
 * @return {string}
 */
const longestPrefix = function(s) {
  return s.slice(0, dfa().pop())
  function dfa() {
    let i = 1
    let j = 0
    const len = s.length
    const prefix = Array(len + 1).fill(0)
    prefix[0] = -1
    prefix[1] = 0
    while(i < len) {
      if(s[j] === s[i]) {
        j++
        i++
        prefix[i] = j
      } else {
        if(j > 0) j = prefix[j]
        else i++
      }
    }
    return prefix
  }  
};