AtCoder Beginner Contest 162 D.RGB Triplets

AtCoder Beginner Contest 162 D.RGB Triplets

题目链接

Problem Statement

We have a string S of length N consisting of R, G, and B.
Find the number of triples $(i,j,k)(1\le i<j<k\le N)$ that satisfy both of the following conditions:

• $S_i\ne S_j,S_i\ne S_k,and{S}_j\ne S_k.$
• $j-i\ne k-j.$

Constraints

• $1\le N\le 4000$
• S is a string of length N consisting of R, G, and B.

Input

Input is given from Standard Input in the following format:
N
S

Output

Print the number of triplets in question.

Sample Input 1

4
RRGB

Sample Output 1

1

Sample Input 2

39
RBRBGRBGGBBRRGBBRRRBGGBRBGBRBGBRBBBGBBB

Sample Output 2

1800

这题首先找所有三元组的个数，然后用循环遍历去掉不符合条件的三元组即可，复杂度 $O((\frac{n}{2}{)}^2)$，AC代码如下：

#include
using namespace std;
typedef long long ll;

int main(){
     
    int len;
    string s;
    cin>>len>>s;
    ll s1=0,s2=0,s3=0,sum=0;
    for(int i=0;i<len;i++){
     
        if(s[i]=='R') sum+=s2*s3,s1++;
        if(s[i]=='G') sum+=s1*s3,s2++;
        if(s[i]=='B') sum+=s1*s2,s3++;
    }
    for(int i=0;i<len;i++){
     
        for(int j=i;j<len;j+=2){
     
            if(s[i]!=s[j] && s[i]!=s[(i+j)/2] && s[j]!=s[(i+j)/2]) sum--;
        }
    }
    cout<<sum;
    return 0;
}