cf#305-A. Mike and Fax-回文串水题

本文解析了CodeForces竞赛中一道关于字符串回文特性的题目。通过算法判断字符串是否可以被拆分为k个等长的回文子串,并提供了一段C++实现代码。

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http://codeforces.com/contest/548/problem/A

题意:

给出一个字符串

给出一个k

问字符串能否拆成k个长度相同的回文串

 首先 k>len 以及  len%k!=0 肯定都是不合法的

然后求出n=len/k,n个回文串,,直接从1开始判断是否每n个字符都是回文串即可。



#include <cstdio>
#include <cmath>
#include <cstring>
#include <string>
#include <algorithm>
#include <iostream>
#include <queue>
#include <map>
#include <set>
#include <vector>
using namespace std;
char tm[1005];

int main()
{ 
    int k;
    int i,j;
        int flag;
    scanf("%s",tm+1);
    int len=strlen(tm+1);
    scanf("%d",&k);
    if (k>len||len%k!=0)
    {
        printf("NO\n");
        return 0;
    }
    int n=len/k;
    for (i=1;i<=len;i+=n)
    {
        int tmp[1005];
        int tm2[1005];
        for (j=1;j<=n;j++)
        {
            tmp[j]=tm[i-1+j];
            tm2[n-j+1]=tmp[j];
        }
          flag=0;
        for (j=1;j<=n;j++)
            if (tm2[j]!=tmp[j])
            {
                flag=1;break;
            }
            if (flag) 
            {
                printf("NO\n");
                return 0;
            }
            
    }
    if (!flag)
    {
        printf("YES\n");
    }
    
    
    
    
    return 0;
    
}

。。
这个配置单有问吗:# dhcpd.conf # # Sample configuration file for ISC dhcpd # # Attention: If /etc/ltsp/dhcpd.conf exists, that will be used as # configuration file instead of this file. # # option definitions common to all supported networks... # option domain-name "example.org"; # option domain-name-servers ns1.example.org, ns2.example.org; default-lease-time 60; max-lease-time 120; # The ddns-updates-style parameter controls whether or not the server will # attempt to do a DNS update when a lease is confirmed. We default to the # behavior of the version 2 packages ('none', since DHCP v2 didn't # have support for DDNS.) ddns-update-style none; # If this DHCP server is the official DHCP server for the local # network, the authoritative directive should be uncommented. authoritative; # Use this to send dhcp log messages to a different log file (you also # have to hack syslog.conf to complete the redirection). #log-facility local7; # No service will be given on this subnet, but declaring it helps the # DHCP server to understand the network topology. #subnet 10.152.187.0 netmask 255.255.255.0 { #} # This is a very basic subnet declaration. subnet 192.168.50.0 netmask 255.255.255.0 { range 192.168.50.40 192.168.50.50; option routers 192.168.50.10; host soho{ hardware ethernet 08:00:07:26:c0:a5; fixed-address 192.168.50.40; } } # This declaration allows BOOTP clients to get dynamic addresses, # which we don't really recommend. #subnet 10.254.239.32 netmask 255.255.255.224 { # range dynamic-bootp 10.254.239.40 10.254.239.60; # option broadcast-address 10.254.239.31; # option routers rtr-239-32-1.example.org; #} # A slightly different configuration for an internal subnet. #subnet 10.5.5.0 netmask 255.255.255.224 { # range 10.5.5.26 10.5.5.30; # option domain-name-servers ns1.internal.example.org; # option domain-name "internal.example.org"; # option subnet-mask 255.255.255.224; # option routers 10.5.5.1; # option broadcast-address 10.5.5.31; # default-lease-time 600; # max-lease-time 7200; #} # Hosts which require special configuration options can be listed in # host statements. If no address is specified, the address will be # allocated dynamically (if possible), but the host-specific information # will still come from the host declaration. #host passacaglia { # hardware ethernet 0:0:c0:5d:bd:95; # filename "vmunix.passacaglia"; # server-name "toccata.example.com"; #} # Fixed IP addresses can also be specified for hosts. These addresses # should not also be listed as being available for dynamic assignment. # Hosts for which fixed IP addresses have been specified can boot using # BOOTP or DHCP. Hosts for which no fixed address is specified can only # be booted with DHCP, unless there is an address range on the subnet # to which a BOOTP client is connected which has the dynamic-bootp flag # set. # You can declare a class of clients and then do address allocation # based on that. The example below shows a case where all clients # in a certain class get addresses on the 10.17.224/24 subnet, and all # other clients get addresses on the 10.0.29/24 subnet. #class "foo" { # match if substring (option vendor-class-identifier, 0, 4) = "SUNW"; #} #shared-network 224-29 { # subnet 10.17.224.0 netmask 255.255.255.0 { # option routers rtr-224.example.org; # } # subnet 10.0.29.0 netmask 255.255.255.0 { # option routers rtr-29.example.org; # } # pool { # allow members of "foo"; # range 10.17.224.10 10.17.224.250; # } # pool { # deny members of "foo"; # range 10.0.29.10 10.0.29.230; # } #}
最新发布
07-25
<think>好的,我需要分析用户提供的DHCP配置文件,并指出其中的问。首先,用户提到静态IP被动态分配了,所以我要检查配置是否正确。 首先看subnet 192.168.50.0的部分。用户配置了range 40到50,同时固定IP地址是40。这里可能存在冲突,因为固定IP在动态分配的范围内,导致服务器可能将40分配给其他客户端。 接着,host声明的位置是否正确?在ISC DHCP配置中,host声明需要放在subnet块内,但根据文档,host可以放在subnet内或外,但最好放在对应子网内以确保正确匹配。用户的host soho确实在subnet块内,这部分没问。 然后检查是否有语法错误。用户配置中的host部分是否正确?比如,fixed-address是否正确指定,是否缺少分号或括号。看起来语法没问。 接下来,用户可能没有排除固定IP。在动态range中包含固定IP会导致冲突。需要将固定IP从range中排除,或者使用deny dynamic bootp clients来阻止动态分配。 另外,default-lease-time和max-lease-time设置为60和120秒,这可能导致IP快速释放,但这不是静态IP被分配的原因。不过较短的租期可能让问更快显现。 检查是否有其他配置覆盖,比如是否有其他subnet或host声明冲突。用户配置中只有一个subnet,其他被注释掉了,所以应该没问。 最后,用户可能没有重启服务或检查日志。需要确认用户是否重启了isc-dhcp-server服务,并检查日志是否有错误。 总结问:固定IP在动态range内,导致冲突。解决方法是调整range排除40,或者在host中添加deny dynamic bootp clients。 </think>您的配置存在以下关键问--- ### **1. 静态IP地址与动态范围重叠** - **问位置**: ```bash range 192.168.50.40 192.168.50.50; fixed-address 192.168.50.40; ``` - **后果**:`192.168.50.40`同时包含在动态分配范围中,可能导致冲突。 - **修复方法**: - **方案1**:调整`range`排除静态IP: ```bash range 192.168.50.41 192.168.50.50; ``` - **方案2**:在`host`块中添加`deny dynamic bootp clients;`: ```bash host soho { hardware ethernet 08:00:07:26:c0:a5; fixed-address 192.168.50.40; deny dynamic bootp clients; } ``` --- ### **2. 子网掩码与网络拓扑匹配性** - **问**:子网`192.168.50.0/24`的掩码为`255.255.255.0`,但网关配置为`192.168.50.10`。需确保: - 网关IP(`192.168.50.10`)在子网范围内。 - 所有客户端的子网掩码一致,否则可能导致通信异常。 --- ### **3. 租约时间过短** - **问**: ```bash default-lease-time 60; max-lease-time 120; ``` - **影响**:动态IP租期过短(60秒),可能导致频繁续约和性能问。建议: ```bash default-lease-time 86400; # 24小时 max-lease-time 172800; # 48小时 ``` --- ### **4. 配置文件优先级问** - **警告**:注释中提到`/etc/ltsp/dhcpd.conf`可能覆盖当前配置。需确认该文件是否存在: ```bash ls /etc/ltsp/dhcpd.conf ``` - 若存在,需同步修改此文件。 --- ### **5. 日志配置缺失** - **建议**:启用日志设施(如`log-facility local7;`),便于排查问: ```bash log-facility local7; ``` - 同时配置`rsyslog`或`syslog`记录该日志。 --- ### **6. 权威模式冲突** - **问**:`authoritative;`指令全局生效,但若网络中存在其他DHCP服务器,可能导致冲突。需确保: - 仅一个DHCP服务器启用此模式。 - 或在`subnet`块内局部启用: ```bash subnet 192.168.50.0 netmask 255.255.255.0 { authoritative; ... } ``` --- ### **修复后的配置示例** ```bash subnet 192.168.50.0 netmask 255.255.255.0 { range 192.168.50.41 192.168.50.50; option routers 192.168.50.10; option domain-name-servers 8.8.8.8; host soho { hardware ethernet 08:00:07:26:c0:a5; fixed-address 192.168.50.40; deny dynamic bootp clients; } } ``` --- ### **验证步骤** 1. **检查语法**: ```bash sudo dhcpd -t -cf /etc/dhcp/dhcpd.conf ``` 2. **重启服务**: ```bash sudo systemctl restart isc-dhcp-server ``` 3. **查看日志**: ```bash sudo tail -f /var/log/syslog | grep dhcpd ``` ---
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