214. Shortest Palindrome

Problem

You are given a string s. You can convert s to a palindrome by adding characters in front of it.

Return the shortest palindrome you can find by performing this transformation.

  Example 1:

Input: s = "aacecaaa"
Output: "aaacecaaa"

Example 2:

Input: s = "abcd"
Output: "dcbabcd"

  Constraints:

• 0 <= s.length <= 5 * 10^4

• s consists of lowercase English letters only.

Solution

/**
 * @param {string} s
 * @return {string}
 */
var shortestPalindrome = function(s) {
  const tmp = `${s}#${reverse(s)}`
  const table = kmp(tmp)
  return `${reverse(s.slice(table[table.length - 1]))}${s}`
};
function reverse(str) {
  return [...str].reverse().join('')
}

function kmp(s) {
  const n = s.length, table = Array(n).fill(0)
  let idx = 0
  for(let i = 1; i < n; ) {
    if(s[i] === s[idx]) {
      idx++
      table[i] = idx
      i++
    } else {
      if(idx > 0) {
        idx = table[idx - 1]
      } else {
        idx = 0
        i++
      }
    }
  }
  return table
}

Explain:

nope.

Complexity:

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