Solved problem 7D from codeforces

Author: AliOsm

CodeForces/7D. Palindrome Degree.cpp

@@ -0,0 +1,86 @@
+/*
+  Idea:
+    - Using hashing we can check for each prefix if it is
+      a palindrome or not.
+    - From the information from the previous point we can
+      calculate the result.
+*/
+
+#include <bits/stdc++.h>
+
+using namespace std;
+
+int const N = 5e6 + 1;
+int const p = 67;
+int const m = 1e9 + 9;
+char s[N];
+int n, res[N];
+long long hash_value[N][2];
+long long p_pow[N];
+
+long long compute_hash() {
+  p_pow[0] = 1;
+  for(int i = 1; i < N; ++i)
+    p_pow[i] = (p_pow[i - 1] * p) % m;
+
+  if(islower(s[0]))
+    hash_value[0][0] = ((s[0] - 'a' + 1) * p_pow[0]) % m;
+  else if(isupper(s[0]))
+    hash_value[0][0] = ((s[0] - 'A' + 27) * p_pow[0]) % m;
+  else
+    hash_value[0][0] = ((s[0] - '0' + 53) * p_pow[0]) % m;
+
+  for(int i = 1; i < n; ++i) {
+    if(islower(s[i]))
+      hash_value[i][0] = (hash_value[i - 1][0] + (s[i] - 'a' + 1) * p_pow[i]) % m;
+    else if(isupper(s[i]))
+      hash_value[i][0] = (hash_value[i - 1][0] + (s[i] - 'A' + 27) * p_pow[i]) % m;
+    else
+      hash_value[i][0] = (hash_value[i - 1][0] + (s[i] - '0' + 53) * p_pow[i]) % m;
+  }
+
+  if(islower(s[n - 1]))
+    hash_value[n - 1][1] = ((s[n - 1] - 'a' + 1) * p_pow[0]) % m;
+  else if(isupper(s[n - 1]))
+    hash_value[n - 1][1] = ((s[n - 1] - 'A' + 27) * p_pow[0]) % m;
+  else
+    hash_value[n - 1][1] = ((s[n - 1] - '0' + 53) * p_pow[0]) % m;
+
+  for(int i = n - 2; i >= 0; --i) {
+    if(islower(s[i]))
+      hash_value[i][1] = (hash_value[i + 1][1] + (s[i] - 'a' + 1) * p_pow[n - i - 1]) % m;
+    else if(isupper(s[i]))
+      hash_value[i][1] = (hash_value[i + 1][1] + (s[i] - 'A' + 27) * p_pow[n - i - 1]) % m;
+    else
+      hash_value[i][1] = (hash_value[i + 1][1] + (s[i] - '0' + 53) * p_pow[n - i - 1]) % m;
+  }
+}
+
+bool is_pal(int idx) {
+  long long first_hash = hash_value[idx][0];
+  long long second_hash = (hash_value[0][1] - hash_value[idx + 1][1] + m) % m;
+
+  first_hash = (first_hash * p_pow[(n - idx - 1)]) % m;
+
+  return first_hash == second_hash;
+}
+
+int main() {
+  scanf("%s", s);
+  n = strlen(s);
+
+  compute_hash();
+
+  res[0] = 1;
+  for(int i = 1; i < n; ++i)
+    if(is_pal(i))
+      res[i] = res[i / 2 - !(i & 1)] + 1;
+
+  int to = 0;
+  for(int i = 0; i < n; ++i)
+    to += res[i];
+  
+  printf("%d\n", to);
+
+  return 0;
+}

CodeForces/README.md

@@ -2,6 +2,7 @@
 
 - [1A. Theatre Square](http://codeforces.com/problemset/problem/1/A)
 - [4A. Watermelon](http://codeforces.com/contest/4/problem/A)
+- [7D. Palindrome Degree](https://codeforces.com/contest/7/problem/D)
 - [9C. Hexadecimal's Numbers](http://codeforces.com/contest/9/problem/C)
 - [12A. Super Agent](http://codeforces.com/contest/12/problem/A)
 - [12B. Correct Solution?](http://codeforces.com/contest/12/problem/B)