Codeforces Round #427 (Div. 2) Problem D Palindromic characteristics (Codeforces 835D) - 记忆化搜索
Palindromic characteristics of string s with length |s| is a sequence of |s| integers, where k-th number is the total number of non-empty substrings of s which are k-palindromes.
A string is 1-palindrome if and only if it reads the same backward as forward.
A string is k-palindrome (k > 1) if and only if:
- Its left half equals to its right half.
- Its left and right halfs are non-empty (k - 1)-palindromes.
The left half of string t is its prefix of length ⌊|t| / 2⌋, and right half — the suffix of the same length. ⌊|t| / 2⌋ denotes the length of string t divided by 2, rounded down.
Note that each substring is counted as many times as it appears in the string. For example, in the string "aaa" the substring "a" appears 3 times.
The first line contains the string s (1 ≤ |s| ≤ 5000) consisting of lowercase English letters.
Print |s| integers — palindromic characteristics of string s.
abba
6 1 0 0
abacaba
12 4 1 0 0 0 0
In the first example 1-palindromes are substring «a», «b», «b», «a», «bb», «abba», the substring «bb» is 2-palindrome. There are no 3- and 4-palindromes here.
题目大意 一个k阶回文串是左右两半的串(长度为这个串的长度除以2向下取整),且这两个串都是k - 1阶回文串。统计一个串内各阶回文串的个数。
表示这道题很简单。。然而比赛时我又把题读错。。。(我这文科渣到了某种境界)。我也不知道我怎么想的,明明看到了half,强行理解成一段。。中间那一段解释half的长度的一段,竟然成功归避。。跑去看样例猜题意了。。。绝望。。。然后不说废话了。
根据数学的直觉和人生的哲理(瞎扯ing),可以知道,你需要用O(n^2)的时间预处理出任意一段子串是否是回文串。
对于一个回文串是否是k阶回文串,因为它自带二分效果(因为一个回文串是对称的,所以如果它的左半边是k - 1阶回文串,那么右半边也一定是),所以可以考虑递归求解。
据说直接求解也可以过。但是个人更喜欢把它写成记忆化搜索吧,可以省掉一个log。
由于一个k阶回文串一定是k - 1阶回文串,所以开始就统计每个子串最高的阶数(如果不是回文串,阶数就当成0吧)的数量,最后再求个后缀和,输答案,完事。
如果还有什么不清楚可以看代码。
Code
/**
* Codeforces
* Problem#835D
* Accepted
* Time: 171ms
* Memory: 124500k
*/
#include <bits/stdc++.h>
using namespace std;
typedef bool boolean; int n;
char s[];
int lvls[][];
boolean vis[][];
int ans[]; inline void init() {
scanf("%s", s + );
n = strlen(s + );
} inline int getLvl(int l, int r) {
if(l == r) return ;
if(lvls[l][r] != || vis[l][r]) return lvls[l][r];
vis[l][r] = true;
int len = (r - l + ) / ;
return lvls[l][r] = (getLvl(l, l + len - ) + );
} inline void solve() {
s[] = '+', s[n + ] = '-', s[n + ] = ;
for(int i = ; i <= n; i++) {
lvls[i][i] = ;
int l = i - , r = i + ;
while(s[l] == s[r]) lvls[l][r] = , l--, r++;
l = i, r = i + ;
while(s[l] == s[r]) lvls[l][r] = , l--, r++;
}
for(int i = ; i <= n; i++)
for(int j = i; j <= n; j++) {
// int c = getLvl(i, j);
// cout << i << " " << j << " " << c << endl;
ans[getLvl(i, j)]++;
}
for(int i = n - ; i; i--)
ans[i] += ans[i + ];
for(int i = ; i <= n; i++)
printf("%d ", ans[i]);
} int main() {
init();
solve();
return ;
}
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