USACO section 1.1 beads

Document directory

• Example
• Program name: beads
• INPUT FORMAT
• Sample input (file beads. in)
• OUTPUT FORMAT
• Sample output (file beads. out)
• OUTPUT EXPLANATION

Broken neck.pdf

You have a neckcycle 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: brbrrrbbbrrrbrrbbrbbbbrrrrb.

Suppose you are to break the neckcycle 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 neckdeskshocould be broken so that the most number of beads can be collected.

Example

For example, for the necktasks 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 encoded 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 neckied.

Program name: beadsINPUT FORMAT

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

Sample input (file beads. in)

29wwwbbrwrbrbrrbrbrwrwwrbwrwrrb

OUTPUT FORMAT

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

Sample output (file beads. out)

11

OUTPUT EXPLANATION

Consider two copies of the beads (kind of like being able to runaround the ends). The string of 11 is marked.

wwwbbrwrbrbrrbrbrwrwwrbwrwrrb wwwbbrwrbrbrrbrbrwrwwrbwrwrrb                       ****** *****

There is a complete necklace consisting of three colors of beads: red, blue, and white. Now, we need to split the beads from a certain place and calculate the number of beads in a continuous color from the broken place, use cnt [] to save and calculate the maximum value of their sum. white can be used as red or blue as needed.

Algorithm Analysis: simulates and adds enumeration. First, the same beads are merged into one node. In this way, the colors of two adjacent nodes are different. Each node is enumerated as the first node of the two nodes for the final sum.

Flag [] is used to mark the color of the current node

　　　　　0 -- blue

　　　　　1 -- red

　　　　　2 -- white

During Processing, cur is used to indicate the color of the current node. When white or cur is encountered, the system exits when the total number of occurrences of the color is greater than 2.

Note: The first and last nodes are adjacent. You need to handle them separately.

Code
  /*
ID: qqxinre1
LANG: C
TASK: beads
*/

#include<stdio.h>
#include<stdlib.h>
#define NN 355

void main()
{
    FILE* fin = fopen("beads.in", "r");
    FILE* fout = fopen("beads.out", "w");

int N, index, ch, i, ans, cur, sum, j, num;
int flag[NN], cnt[NN];
char str[NN];
    fscanf(fin, "%d%s", &N, str);
    index = -1;
for (i = 0; i < N; i++){
if (str[i] == 'b')
            ch = 0;
else if (str[i] == 'r')
            ch = 1;
else
            ch = 2;
if (i == 0 || str[i] != str[i - 1]){
            cnt[++index] = 1;
            flag[index] = ch;
        }
else
            cnt[index]++;
    }

if (index == 0)
    {
        fprintf(fout, "%d\n", cnt[0]);
        exit(0);
    }
if (flag[0] == flag[index])
        cnt[0] += cnt[index--];

    ans = 0;
for (i = 0; i <= index; i++){
        cur = -1;
        sum = 0;
for (j = i; j <= index; j++){
if (flag[j] == 2){
                sum += cnt[j];
continue;
            }
if (cur == -1){
                cur = flag[j];
                num = 1;
            }
else{
if (cur != flag[j]){
                    cur = flag[j];
                    num ++;
if (num > 2)
break;
                }
            }
            sum += cnt[j];
        }

if (j > index){
for (j = 0; j < i; j++){
if (flag[j] == 2){
                    sum += cnt[j];
continue;
                }
if (cur == -1){
                    cur = flag[j];
                    num = 1;
                }
else{
if (cur != flag[j]){
                        cur = flag[j];
                        num ++;
if (num > 2)
break;
                    }
                }
                sum += cnt[j];
            }
        }
if (sum > ans)
            ans = sum;
    }
    fprintf(fout, "%d\n", ans);
    exit(0);
}