Add string/manacher.cc

Signed-off-by: subcrip <contact@subc.rip>

string/manacher.cc

@@ -0,0 +1,28 @@
+template <typename T>
+vector<int> manacher(const vector<T>& s, const T& never) {
+    vector<T> t = { never };
+    for (auto&& c : s) {
+        t.emplace_back(c);
+        t.emplace_back(never);
+    }
+    int n = t.size();
+    vector<int> r(n);
+    for (int i = 0, j = 0; i < n; ++i) {
+        if (2 * j - i >= 0 && j + r[j] > i) {
+            r[i] = min(r[2 * j - i], j + r[j] - i);
+        }
+        while (i - r[i] >= 0 && i + r[i] < n && t[i - r[i]] == t[i + r[i]]) {
+            r[i] += 1;
+        }
+        if (i + r[i] > j + r[j]) {
+            j = i;
+        }
+    }
+    for (int i = 0; i < n; i += 2) {
+        r[i] = (r[i] - 1) / 2;
+    }
+    for (int i = 1; i < n; i += 2) {
+        r[i] /= 2;
+    }
+    return r;
+}