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
1 /** 2 * Codeforces 3 * Problem#835D 4 * Accepted 5 * Time: 171ms 6 * Memory: 124500k 7 */ 8 #include <bits/stdc++.h> 9 using namespace std; 10 typedef bool boolean; 11 12 int n; 13 char s[5005]; 14 int lvls[5002][5002]; 15 boolean vis[5002][5002]; 16 int ans[5005]; 17 18 inline void init() { 19 scanf("%s", s + 1); 20 n = strlen(s + 1); 21 } 22 23 inline int getLvl(int l, int r) { 24 if(l == r) return 1; 25 if(lvls[l][r] != 1 || vis[l][r]) return lvls[l][r]; 26 vis[l][r] = true; 27 int len = (r - l + 1) / 2; 28 return lvls[l][r] = (getLvl(l, l + len - 1) + 1); 29 } 30 31 inline void solve() { 32 s[0] = '+', s[n + 1] = '-', s[n + 2] = 0; 33 for(int i = 1; i <= n; i++) { 34 lvls[i][i] = 1; 35 int l = i - 1, r = i + 1; 36 while(s[l] == s[r]) lvls[l][r] = 1, l--, r++; 37 l = i, r = i + 1; 38 while(s[l] == s[r]) lvls[l][r] = 1, l--, r++; 39 } 40 for(int i = 1; i <= n; i++) 41 for(int j = i; j <= n; j++) { 42 // int c = getLvl(i, j); 43 // cout << i << " " << j << " " << c << endl; 44 ans[getLvl(i, j)]++; 45 } 46 for(int i = n - 1; i; i--) 47 ans[i] += ans[i + 1]; 48 for(int i = 1; i <= n; i++) 49 printf("%d ", ans[i]); 50 } 51 52 int main() { 53 init(); 54 solve(); 55 return 0; 56 }