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≤i<j<k≤N)(i, j, k) \ (1≤i<j<k≤N)(i,j,k) (1≤i<j<k≤N) that satisfy both of the following conditions:

Si≠Sj,Si≠Sk,andSj≠Sk.S_i≠S_j, S_i≠S_k, and S_j≠S_k.Si=Sj,Si=Sk,andSj=Sk.
j−i≠k−j.j−i≠k−j.j−i=k−j.

Constraints

1≤N≤40001≤N≤40001≤N≤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

Sample Input 2

39
RBRBGRBGGBBRRGBBRRRBGGBRBGBRBGBRBBBGBBB

Sample Output 2

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((n2)2)O((\frac{n}{2})^2)O((2n)2)，AC代码如下：

#include<bits/stdc++.h>
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;
}

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AtCoder Beginner Contest 162 D.RGB Triplets

AtCoder Beginner Contest 162 D.RGB Triplets

Problem Statement

Constraints

Input

Output

Sample Input 1

Sample Output 1

Sample Input 2

Sample Output 2

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