Implement strStr()

Description

Implement strStr().

Return the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.

Example 1:

Input: haystack = "hello", needle = "ll"
Output: 2

Example 2:

Input: haystack = "aaaaa", needle = "bba"
Output: -1

Clarification:

What should we return when needle is an empty string? This is a great question to ask during an interview.

For the purpose of this problem, we will return 0 when needle is an empty string. This is consistent to C's strstr() and Java's indexOf().

 

Constraints:

• haystack and needle consist only of lowercase English characters.

Solution(javascript)

/** Rabin-Karp
 * @param {string} haystack
 * @param {string} needle
 * @return {number}
 */
const strStr = function (haystack, needle) {
  const base = 256
  const prime = 1439173969
  if (needle === '') {
    return 0
  }
  let hashNeedle = 0
  const magic = (base ** (needle.length - 1)) % prime
  const product = prime * base
  for (const c of needle) {
    hashNeedle = (hashNeedle * base + c.charCodeAt(0)) % prime
  }
  const isEqual = i => needle === haystack.slice(i, i + needle.length)
  let hash = 0
  for (let i = 0; i <= haystack.length - needle.length; i++) {
    if (i === 0) {
      for (let j = 0; j < needle.length; j++) {
        hash = (hash * base + haystack[j].charCodeAt(0)) % prime
      }
    } else {
      hash = (hash - haystack[i - 1].charCodeAt(0) * magic + product) % prime
      hash = (hash * base + haystack[i + needle.length - 1].charCodeAt(0)) % prime
    }
    if (hash === hashNeedle && isEqual(i)) {
      return i
    }
  }
  return -1
}