647. Palindromic Substrings

Return the number of palindromic substrings in s. Substrings at different positions count separately.

Every successful expansion step from one of the 2n − 1 centers is exactly one palindrome. Count them.

1function countSubstrings(s: string): number {
2  let count = 0;
3  const expand = (l: number, r: number) => {
4    while (l >= 0 && r < s.length && s[l] === s[r]) { count++; l--; r++; }
5  };
6  for (let i = 0; i < s.length; i++) {
7    expand(i, i);
8    expand(i, i + 1);
9  }
10  return count;
11}

count = 0

s

Centers at 0 and 0|1.

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