LeetCode: Palindromic Substrings Solution

1/**
2 * @param {string} s
3 * @return {number}
4 */
5var countSubstrings = function (s) {
6  const N = s.length
7  const mem = Array.from({ length: N }, _ => Array(N).fill(null))
8
9  const isPalindrome = (i, j) => {
10    if (mem[i][j] !== null) {
11      return mem[i][j]
12    }
13
14    if (i >= j) {
15      return (mem[i][j] = 1)
16    }
17
18    if (s[i] !== s[j]) {
19      return (mem[i][j] = 0)
20    }
21
22    return (mem[i][j] = isPalindrome(i + 1, j - 1))
23  }
24
25  let res = 0
26
27  for (let i = 0; i < N; i++) {
28    for (let j = i; j < N; j++) {
29      res += isPalindrome(i, j) ? 1 : 0
30    }
31  }
32
33  return res
34}