solve: palindromic substrings

Author: tolluset

palindromic-substrings/tolluset.ts

@@ -0,0 +1,59 @@
+/*
+ * TC: O(n)
+ * SC: O(n)
+ * */
+function countSubstrings(s: string): number {
+  const t = "#" + s.split("").join("#") + "#";
+  const n = t.length;
+  const p = new Array(n).fill(0);
+  let center = 0,
+    right = 0,
+    l = 0;
+
+  const mirror = (i: number, p: number[], c: number, r: number) => {
+    return Math.min(r - i, p[c * 2 - i]);
+  };
+
+  const isRightBound = (i: number, p: number[], n: number) => {
+    return i + p[i] + 1 < n;
+  };
+
+  const isLeftBound = (i: number, p: number[]) => {
+    return i - p[i] - 1 >= 0;
+  };
+
+  const isPalindrome = (i: number, p: number[], t: string) => {
+    return t[i + p[i] + 1] === t[i - p[i] - 1];
+  };
+
+  const isLongest = (i: number, p: number[], r: number) => {
+    return i + p[i] > r;
+  };
+
+  const calcTotal = (i: number, p: number[]) => {
+    return Math.floor((p[i] + 1) / 2);
+  };
+
+  for (let i = 0; i < n; i++) {
+    if (i < right) {
+      p[i] = mirror(i, p, center, right);
+    }
+
+    while (
+      isRightBound(i, p, n) &&
+      isLeftBound(i, p) &&
+      isPalindrome(i, p, t)
+    ) {
+      p[i]++;
+    }
+
+    if (isLongest(i, p, right)) {
+      center = i;
+      right = i + p[i];
+    }
+
+    l += calcTotal(i, p);
+  }
+
+  return l;
+}