# USACO Section 1.1.7 Broken Necklace

## 题目

（由于USACO在外部无法直接访问题库，所以把题目贴上，可以通过上面的目录跳过）

# Broken Necklace

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


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 will include 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 FORMAT

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

29
wwwbbrwrbrbrrbrbrwrwwrbwrwrrb


### OUTPUT FORMAT

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

11


### OUTPUT EXPLANATION

Consider two copies of the beads (kind of like being able to runaround the ends). The string of 11 is marked.
                       original 'split'
v
wwwbbrwrbrbrrbrbrwrwwrbwrwrrb|wwwbbrwrbrbrrbrbrwrwwrbwrwrrb
******|*****
rrrrrb|bbbbb  <-- assignments
5 x r  6 x b  <-- 11 total

## 思路

### 换个神似靠谱的法子吧

1.跟前面一样或者自己是白色的，直接挂上；

2.剩下的就是另一种颜色了（既跟前面不一样又还不是白的）。这样的话首先要判断前面是不是白色的

1）如果是白色的还得把前面的颜色一样的也挂。

## 代码

#include <cstdio>
char a[777];
int l[777],r[777],n;
int main(){
scanf("%d%s",&n,a);
for (int i(0);i<n;i++){
a[n+i]=a[i];
}
int i=0,j=a[0];
while (++i<2*n){
if ( a[i]==j || a[i]=='w' ){
l[i]=l[i-1]+1;
}
else {
if (a[i-1]=='w'){
int k=i-1;
while (k>=0 && a[k]=='w'){k--;}
l[i]=i-k-1;
}
j=a[i];
}
}
i=2*n-1;
j=a[2*n-1];
while (--i>=0){
if ( a[i]==j || a[i]=='w'){
r[i]=r[i+1]+1;
}
else {
if (a[i+1]=='w'){
int k=i+1;
while (k<2*n && a[k]=='w'){k++;}
r[i]=k-i-1;
}
j=a[i];
}
}
for (int i(0);i<n;i++) l[i]=l[i+n];
int ans=0;
for (int i(0);i<n-1;i++){
if (l[i]+r[i+1]>ans) ans=l[i]+r[i+1];
}
if (ans>n){
printf("%d\n",n);
} else {
printf("%d\n",ans+2);
}
return 0;
}