The “U.S. Robots” HQ has just received a rather
alarming anonymous letter. It states that the agent from the competing ?Robots
Unlimited? has infiltrated into “U.S. Robotics”. ?U.S. Robots? security service
would have already started an undercover operation to establish the agent’s
identity, but, fortunately, the letter describes communication channel the agent
uses. He will publish articles containing stolen data to the “Solaris” almanac.
Obviously, he will obfuscate the data, so “Robots Unlimited” will have to use a
special descrambler (“Robots Unlimited” part number NPRx8086, specifications are
kept secret).
Having read the letter, the “U.S. Robots”
president recalled having hired the “Robots Unlimited” ex-employee John Pupkin.
President knows he can trust John, because John is still angry at being
mistreated by “Robots Unlimited”. Unfortunately, he was fired just before his
team has finished work on the NPRx8086 design.
So, the president has assigned the task of
agent’s message interception to John. At first, John felt rather embarrassed,
because revealing the hidden message isn’t any easier than finding a needle in a
haystack. However, after he struggled the problem for a while, he remembered
that the design of NPRx8086 was still incomplete. “Robots Unlimited” fired John
when he was working on a specific module, the text direction detector. Nobody
else could finish that module, so the descrambler will choose the text scanning
direction at random. To ensure the correct descrambling of the message by
NPRx8086, agent must encode the information in such a way that the resulting
secret message reads the same both forwards and backwards.
In addition, it is reasonable to assume that the agent will be sending a very long message, so John has simply to find the longest message satisfying the mentioned property.
In addition, it is reasonable to assume that the agent will be sending a very long message, so John has simply to find the longest message satisfying the mentioned property.
Your task is to help John Pupkin by writing a
program to find the secret message in the text of a given article. As NPRx8086
ignores white spaces and punctuation marks, John will remove them from the text
before feeding it into the program.
Input
The input consists of a single line, which
contains a string of Latin alphabet letters (no other characters will appear in
the string). String length will not exceed 1000 characters.
Output
The longest substring with mentioned property.
If there are several such strings you should output the first of
them.
题目大意：求最长回文子串并输出。
思路：Manacher算法的模板题，不多说。
代码（0.015S）：
1 #include <cstdio> 2 #include <algorithm> 3 #include <cstring> 4 #include <iostream> 5 using namespace std; 6 typedef long long LL; 7 8 const int MAXN = 2 * 1010; 9 10 char str[MAXN], s[MAXN]; 11 int n, p[MAXN]; 12 13 void manacher() { 14 int mx = 0, id; 15 for(int i = 1; i < n; ++i) { 16 if(mx > i) p[i] = min(p[2 * id - i], mx - i); 17 else p[i] = 1; 18 while(s[i + p[i]] == s[i - p[i]]) ++p[i]; 19 if(i + p[i] > mx) { 20 id = i; 21 mx = i + p[i]; 22 } 23 } 24 } 25 26 void print() { 27 int id = 0; 28 for(int i = 1; i < n; ++i) 29 if(p[i] > p[id]) id = i; 30 int l = (id - p[id] + 1) / 2, r = l + p[id] - 1; 31 for(int i = l; i < r; ++i) putchar(str[i]); 32 puts(""); 33 } 34 35 int main() { 36 scanf("%s", str); 37 s[n++] = ‘$‘, s[n++] = ‘#‘; 38 for(int i = 0; str[i]; ++i) { 39 s[n++] = str[i]; 40 s[n++] = ‘#‘; 41 } 42 s[n] = 0; 43 manacher(); 44 print(); 45 }