🧑‍💻 improves manacher implementation

algorithms/strings/longest-palindrome-substring-(manacher).cpp

@@ -1,44 +1,37 @@
+const ll oo = 1e18;
 vi manacher(const string &s) {
   string t2;
-  for (auto c : s) t2 += string("#") + c;
-  t2 = t2 + '#';
-  int n = t2.size();
-  t2 = "$" + t2 + "^";
+  t2.push_back('$');
+  for (auto c : s) {
+    t2.push_back('#');
+    t2.push_back(c);
+  }
+  t2.push_back('#');
+  t2.push_back('^');
+  int n = len(t2) - 2;  // bc of $ and ^
   vi p(n + 2);
   int l = 1, r = 1;
   for (int i = 1; i <= n; i++) {
     p[i] = max(0, min(r - i, p[l + (r - i)]));
-    while (t2[i - p[i]] == t2[i + p[i]]) {
-      p[i]++;
-    }
-    if (i + p[i] > r) {
-      l = i - p[i], r = i + p[i];
-    }
+    while (t2[i - p[i]] == t2[i + p[i]]) p[i]++;
+    if (i + p[i] > r) l = i - p[i], r = i + p[i];
     p[i]--;
   }
-  return vi(begin(p) + 1, end(p) - 1);
+  return vi(p.begin() + 1, p.end() - 1);
 }
 string longest_palindrome(const string &s) {
   vi xs = manacher(s);
 
   string s2;
-  for (auto c : s) s2 += string("#") + c;
-  s2 = s2 + '#';
+  for (auto c : s) s2.push_back('#'), s2.push_back(c);
+  s2.push_back('#');
 
-  // refactor this to use cpp max and iterators arithmetic
-  int mpos = 0;
-  for (int i = 0; i < len(xs); i++) {
-    if (xs[i] > xs[mpos]) {
-      mpos = i;
-    }
-  }
+  int mpos = max_element(all(xs)) - xs.begin();
 
   string ans;
   int k = xs[mpos];
-  for (int i = mpos - k; i <= mpos + k; i++) {
-    if (s2[i] != '#') {
-      ans += s2[i];
-    }
-  }
+  for (int i = mpos - k; i <= mpos + k; i++)
+    if (s2[i] != '#') ans.push_back(s2[i]);
+
   return ans;
 }

submissions/CSES/longest-palindrome.cpp

@@ -1,4 +1,6 @@
 #include <bits/stdc++.h>
+
+#include <algorithm>
 using namespace std;
 #define endl '\n'
 #define fastio                      \
@@ -25,49 +27,43 @@ using vc = vector<char>;
 #define eb(___x) emplace_back(___x)
 
 const ll oo = 1e18;
-vi manacher(string s) {
+vi manacher(const string &s) {
   string t2;
-  for (auto c : s) t2 += string("#") + c;
-  t2 = t2 + '#';
-  int n = t2.size();
-  t2 = "$" + t2 + "^";
+  t2.push_back('$');
+  for (auto c : s) {
+    t2.push_back('#');
+    t2.push_back(c);
+  }
+  t2.push_back('#');
+  t2.push_back('^');
+  int n = len(t2) - 2;  // bc of $ and ^
   vi p(n + 2);
   int l = 1, r = 1;
   for (int i = 1; i <= n; i++) {
     p[i] = max(0, min(r - i, p[l + (r - i)]));
-    while (t2[i - p[i]] == t2[i + p[i]]) {
-      p[i]++;
-    }
-    if (i + p[i] > r) {
-      l = i - p[i], r = i + p[i];
-    }
+    while (t2[i - p[i]] == t2[i + p[i]]) p[i]++;
+    if (i + p[i] > r) l = i - p[i], r = i + p[i];
     p[i]--;
   }
-  return vi(begin(p) + 1, end(p) - 1);
+  return vi(p.begin() + 1, p.end() - 1);
 }
 string longest_palindrome(const string &s) {
   vi xs = manacher(s);
 
   string s2;
-  for (auto c : s) s2 += string("#") + c;
-  s2 = s2 + '#';
+  for (auto c : s) s2.push_back('#'), s2.push_back(c);
+  s2.push_back('#');
 
-  int mpos = 0;
-  for (int i = 0; i < len(xs); i++) {
-    if (xs[i] > xs[mpos]) {
-      mpos = i;
-    }
-  }
+  int mpos = max_element(all(xs)) - xs.begin();
 
   string ans;
   int k = xs[mpos];
-  for (int i = mpos - k; i <= mpos + k; i++) {
-    if (s2[i] != '#') {
-      ans += s2[i];
-    }
-  }
+  for (int i = mpos - k; i <= mpos + k; i++)
+    if (s2[i] != '#') ans.push_back(s2[i]);
+
   return ans;
 }
+
 void run() {
   string s;
   cin >> s;