USACO 1.1 Broken Necklace

题目连接

http://train.usaco.org/usacoprob2?a=XoNUWPzYQSw&S=beads

Description

You have a necklace of N red, white, or blue beads (3<=N<=350) some of which are red, others blue, and others white, arranged at random. Here are two examples for n=29:

            1 2                               1 2
        r b b r                           b r r b
      r         b                       b         b
     r           r                     b           r
    r             r                   w             r
   b               r                 w               w
  b                 b               r                 r
  b                 b               b                 b
  b                 b               r                 b
   r               r                 b               r
    b             r                   r             r
     b           r                     r           r
       r       r                         r       b
         r b r                             r r w
        Figure A                         Figure B
                    r red bead
                    b blue bead
                    w white bead

The beads considered first and second in the text that follows have been marked in the picture.

The configuration in Figure A may be represented as a string of b’s and r’s, where b represents a blue bead and r represents a red one, as follows: brbrrrbbbrrrrrbrrbbrbbbbrrrrb .

Suppose you are to break the necklace at some point, lay it out straight, and then collect beads of the same color from one end until you reach a bead of a different color, and do the same for the other end (which might not be of the same color as the beads collected before this).

Determine the point where the necklace should be broken so that the most number of beads can be collected.

Example

For example, for the necklace in Figure A, 8 beads can be collected, with the breaking point either between bead 9 and bead 10 or else between bead 24 and bead 25.

In some necklaces, white beads had been included as shown in Figure B above. When collecting beads, a white bead that is encountered may be treated as either red or blue and then painted with the desired color. The string that represents this configuration can include any of the three symbols r, b and w.

Write a program to determine the largest number of beads that can be collected from a supplied necklace.

Input

Line 1: N, the number of beads
Line 2: a string of N characters, each of which is r, b, or w

Output

A single line containing the maximum of number of beads that can be collected from the supplied necklace.

Sample Input

29
wwwbbrwrbrbrrbrbrwrwwrbwrwrrb

Sample Output

11

题意

从某一个端点断开项链,然后从断开的两端数珠子,直到第一个与数的珠子颜色不同的珠子,停止计数。其中W颜色可以染色成R颜色或者B颜色。

题解

虽然说是水题,但是这题实在是不会啊,代码写得很乱很难看啊,只能过过样例啊,只好看官方题解了,在参数多于两个的时候应该调用函数封装好,比如本题的数珠子的方向dir与断点i。还有取余的模拟环操作也要注意判断是否为负,为负加一个len就可以了。

代码

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/*
ID: hyson601
PROG: beads
LANG: C++
*/
#include <iostream>
#include <fstream>
using namespace std;
const int maxn = 1e5+10;
char s[maxn];
int len;
int mod(int n,int m){
while(n<0) n+=m;
return n%m;
}
int tobreak(int p,int dir){
int i;
int n;
if(dir>0) i=p;
else i=mod(p-1,len);
char color='w';
for(n=0;n<len;i=mod(i+dir,len)){
if(color=='w' && s[i]!='w')
color=s[i];
else if(color!=s[i] && s[i]!='w')
break;
++n;
}
return n;
}
int main(){
freopen("beads.in","r",stdin);
freopen("beads.out","w",stdout);
scanf("%d",&len);
scanf("%s",s);
int n,m=0;
for(int i=0;i<len;++i){
n=tobreak(i,1)+tobreak(i,-1);
if(n>m) m=n;
}
if(m>len) m=len;
printf("%d\n",m);
return 0;
}