K、 floyd模板

#include<bits/stdc++.h>
using namespace std;
#define ll long long
int const N=107;
int n,m,q,s,t,cost;
ll e[N][N];
ll pan(int num,int d,int tol){
	ll z=1,res=0;
	for(int i=1;i<=d;++i){
		z*=num;
		res+=z;
		if(res>tol) return res;
	}
	return res;
}
int main(){
	cin >> n >> m;
	memset(e,0x3f,sizeof e);  //极大值用3f 
	for(int i=1;i<=m;++i){
		int a,b;cin >> a >> b;
		e[a][b]=e[b][a]=1;
	}
	for(int k=1;k<=n;++k){ //中间点在最外层
		for(int i=1;i<=n;++i){
			for(int j=1;j<=n;++j){
				//if(i==j) e[i][j]=e[j][i]=0;
				if(e[i][k]+e[k][j]<e[i][j]){
					e[i][j]=e[i][k]+e[k][j];
				}
			}
		} 
	}
	cin >> q;
	while(q--){
		cin >> s >> t >> cost;
		ll l=0,r=1e9;
		while(l<=r){
			ll mid=(l+r) >> 1;
			ll z=pan(mid,e[s][t],cost);
			if(z<=cost) l=mid+1;
			else r=mid-1; 
		}
		cout << l-1 << "\n";
	}
	return 0;
}

E 异或前缀和

子序列的问题大部分不是dp就是字符串 子串的问题还有可能是双指针

#include<bits/stdc++.h>
using namespace std;
#define ll long long
int const N=1e6+7;
int n,a[N],sum[N];
map<int,int>mp;
int main(){
	cin >> n;
	ll ans=0;mp[0]=1;
	for(int i=1;i<=n;++i){
		cin >> a[i];
		sum[i]=sum[i-1]^(1<<a[i]);
		ans+=mp[sum[i]];
		mp[sum[i]]++;
	}
	cout << ans;
	return 0;
	
}