Luogu P2922 秘密消息

原题

P2922 [USACO08DEC]秘密消息Secret Message

题目描述

Bessie is leading the cows in an attempt to escape! To do this, the cows are sending secret binary messages to each other.

Ever the clever counterspy, Farmer John has intercepted the first b_i (1 <= b_i <= 10,000) bits of each of M (1 <= M <= 50,000) of these secret binary messages.

He has compiled a list of N (1 <= N <= 50,000) partial codewords that he thinks the cows are using. Sadly, he only knows the first c_j (1 <= c_j <= 10,000) bits of codeword j.

For each codeword j, he wants to know how many of the intercepted messages match that codeword (i.e., for codeword j, how many times does a message and the codeword have the same initial bits). Your job is to compute this number.

The total number of bits in the input (i.e., the sum of the b_i and the c_j) will not exceed 500,000.

Memory Limit: 32MB

POINTS: 270

贝茜正在领导奶牛们逃跑．为了联络，奶牛们互相发送秘密信息．

信息是二进制的，共有M(1≤M≤50000)条．反间谍能力很强的约翰已经部分拦截了这些信息，知道了第i条二进制信息的前bi(l《bi≤10000)位．他同时知道，奶牛使用N(1≤N≤50000)条密码．但是，他仅仅了解第J条密码的前cj(1≤cj≤10000)位．

对于每条密码J，他想知道有多少截得的信息能够和它匹配．也就是说，有多少信息和这条密码有着相同的前缀．当然，这个前缀长度必须等于密码和那条信息长度的较小者．

在输入文件中，位的总数（即∑Bi+∑Ci）不会超过500000.

输入输出格式

输入格式：

 

* Line 1: Two integers: M and N

* Lines 2..M+1: Line i+1 describes intercepted code i with an integer b_i followed by b_i space-separated 0's and 1's

* Lines M+2..M+N+1: Line M+j+1 describes codeword j with an integer c_j followed by c_j space-separated 0's and 1's

 

输出格式：

 

* Lines 1..M: Line j: The number of messages that the jth codeword could match.

 

输入输出样例

输入样例#1： 
 
4 5 
3 0 1 0 
1 1 
3 1 0 0 
3 1 1 0 
1 0 
1 1 
2 0 1 
5 0 1 0 0 1 
2 1 1

　　

输出样例#1： 
 
1 
3 
1 
1 
2

　　

说明

Four messages; five codewords.

The intercepted messages start with 010, 1, 100, and 110.

The possible codewords start with 0, 1, 01, 01001, and 11.

0 matches only 010: 1 match

1 matches 1, 100, and 110: 3 matches

01 matches only 010: 1 match

01001 matches 010: 1 match

11 matches 1 and 110: 2 matches

 

题目大意：

约翰拦截到了奶牛的密码，并且他有一个密码本，里面不完全的记录了奶牛逃跑用的密码，他要从里面比对出密码。

密码都是二进制的，为0或1。

首先我们从样例里面就能分析出这个题的坑点。

看第二组的第5个二进制串，很显然它比trie树中的二进制串都要长，如果按照一般的trie来写，是找不出它的前缀的。

所以我们要在结构体中添加一个成员sign。

那么sign是干什么的呢？

它是用来标记这个二进制串是否是一个完整的串。注意要用int形的变量。因为可能用完全相同的串出现在树中。

而我们只需要在insert操作时加上后面这一句

 

rt->sign++;

 

把find变成下面的样子

int find(char *s) {
	node *rt = root;
	int k = 0;
	bool mark = false;
	for(int i=0; i<len; i++) {
		int id = s[i]-'0';
		if(rt->next[id] == NULL) {
			mark = true;
			break;
		}
		rt = rt->next[id];
		k += rt->sign;
	}
	if(mark == false) k+=rt->count-rt->sign;
	return k;
}

就可以啦

完整代码

#include <iostream>
#include <cstdio>
#include <cstring>

using namespace std;

struct node{
    int count;
    node *next[5];
    int sign;
}*root;
int n, m, len;
char s[10008];

node *build() {
    node *k = new(node);
    k->count = 0;
    k->sign = 0;
    memset(k->next, NULL, sizeof(k->next));
    return k;
}

void insert(char *s) {
    node *rt = root;
    for(int i=0; i<len; i++) {
        int id = s[i]-'0';
        if(rt->next[id] == NULL) {
            rt->next[id] = build();
        }
        rt = rt->next[id];
        rt->count++;
    }
    rt->sign++;
}

int find(char *s) {
    node *rt = root;
    int k = 0;
    bool mark = false;
    for(int i=0; i<len; i++) {
        int id = s[i]-'0';
        if(rt->next[id] == NULL) {
            mark = true;
            break;
        }
        rt = rt->next[id];
        k += rt->sign;
    }
    if(mark == false) k+=rt->count-rt->sign;
    return k;
}

int main() {
    root = build();
    scanf("%d%d", &n, &m);
    for(int i=1; i<=n; i++) {
        memset(s, NULL, sizeof(s));
        scanf("%d", &len);
        for(int j=0; j<len; j++) {
            int k;
            scanf("%d", &k);
            if(k == 0) s[j] = '0';
            if(k == 1) s[j] = '1';
        }
        insert(s);
    }
    for(int i=1; i<=m; i++) {
        memset(s, NULL, sizeof(s));
        scanf("%d", &len);
        for(int j=0; j<len; j++) {
            int k;
            scanf("%d", &k);
            if(k == 0) s[j] = '0';
            if(k == 1) s[j] = '1';
        }
        int ans = find(s);
        printf("%d\n", ans);
    }
}

 

作者：wlz
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posted @ 2018-04-03 18:50  Mystical-W  阅读(280)  评论(2)  编辑  收藏  举报