1392. Longest Happy Prefix

Description

A string is called a happy prefix if is a non-empty prefix which is also a suffix (excluding itself).

Given a string s, return the longest happy prefix of s. Return an empty string "" if no such prefix exists.

 

Example 1:

Input: s = "level"
Output: "l"
Explanation: s contains 4 prefix excluding itself ("l", "le", "lev", "leve"), and suffix ("l", "el", "vel", "evel"). The largest prefix which is also suffix is given by "l".

Example 2:

Input: s = "ababab"
Output: "abab"
Explanation: "abab" is the largest prefix which is also suffix. They can overlap in the original string.

 

Constraints:

• 1 <= s.length <= 105
• s contains only lowercase English letters.

 

Solutions

Solution: Longest Prefix Suffix

• Time complexity: O(n)
• Space complexity: O(n)

 

JavaScript

/**
 * @param {string} s
 * @return {string}
 */
const longestPrefix = function (s) {
  const n = s.length;
  const lps = Array.from({ length: n }, () => 0);
  let length = 0;

  for (let index = 1; index < n; index++) {
    while (length && s[index] !== s[length]) {
      length = lps[length - 1];
    }
    if (s[index] === s[length]) {
      length += 1;
    }
    lps[index] = length;
  }
  return s.slice(0, lps[n - 1]);
};