LeetCode: Permutation in String Solution

Approach

Notice that there's a constraint "s1 and s2 consist of lowercase English letters"

Maintain a sliding window with size of s1.length

Use a counter base on character occurences to detect for permutation

Question now turns to "check if two strings are permutation of each other"

Implementation

1var checkInclusion = function (s1, s2) {
2  let [n1, n2] = [s1.length, s2.length]
3  let occurences = Array(26).fill(0)
4
5  const mutate = (charCode, dir) =>
6    (occurences[charCode - "a".charCodeAt(0)] += dir)
7
8  const isMatch = () => occurences.every(x => x === 0)
9
10  for (let i = 0; i < n1; i++) {
11    mutate(s1.charCodeAt(i), +1)
12  }
13
14  for (let i = 0; i < n2; i++) {
15    mutate(s2.charCodeAt(i), -1)
16    if (i - n1 >= 0) mutate(s2.charCodeAt(i - n1), +1)
17
18    if (isMatch()) return true
19  }
20
21  return false
22}