Add Manacher's algorithm

Author: coderick14

manacher.cpp

@@ -0,0 +1,58 @@
+#include <bits/stdc++.h>
+using namespace std;
+
+string getPaddedString(const string& s, string delim = "#") {
+	string paddedString = "#";
+	for (const char& c : s) {
+		paddedString += c;
+		paddedString += delim;
+	}
+	return paddedString;
+}
+
+long getMaxPos(const vector<long>& span) {
+	long maxlen = 1, pos = 1, n = span.size();
+	for (long i = 2; i < n; i++) {
+		if (span[i] > maxlen) {
+			maxlen = span[i];
+			pos = i;
+		}
+	}
+	return pos;
+}
+
+string getUnpaddedString(const string& s, const long& maxPos, const long& maxlen) {
+	string ans;
+	long start = maxPos - maxlen + 1, end = maxPos + maxlen - 1;
+	for (long i = start; i <= end; i++) {
+		if (s[i] != '#') ans += s[i];
+	}
+	return ans;
+}
+
+string manacher(const string& s) {
+	string str = getPaddedString(s);
+	long n = str.length();
+	vector<long> span(n,1);
+	long c = 0, r = 0;
+	for (long i = 1; i < n; i++) {
+		if (i <= r and 2*c - i >= 0) {
+			span[i] = min(r - i + 1, span[2*c - i]);
+		}
+		while (i - span[i] >= 0 and i + span[i] < n and str[i - span[i]] == str[i+span[i]]) span[i]++;
+		if (i + span[i] - 1 > r) {
+			c = i;
+			r = i + span[i] - 1;
+		}
+	}
+	long maxPos = getMaxPos(span);
+	long maxlen = span[maxPos];
+	return getUnpaddedString(str, maxPos, maxlen);
+}
+
+int main() {
+	string s;
+	cin >> s;
+	cout << manacher(s) << endl;
+	return 0;
+}