题目链接:https://ac.nowcoder.com/acm/contest/3002/F
题解思路:
思路统计连通块白点的个数(可用并查集,或者搜索)
- 搜索: https://ac.nowcoder.com/acm/contest/view-submission?submissionId=42763870
- 并查集: https://blog.csdn.net/qq_42815188/article/details/104220706
代码
#include <bits/stdc++.h>
using namespace std;
const int MAXN = 100005;
vector<vector<int> > mp;
int vis[MAXN], num[MAXN];
vector<int> tmp;
int bfs(int now) {
queue<int> q;
q.push(now);
tmp.push_back(now);
vis[now] = 1;
int cnt = 1; // now 也算一个
while(!q.empty()) {
int k = q.front();
q.pop();
int vsize = mp[k].size();
for(int i = 0; i < vsize; i++){
if(vis[mp[k][i]] == 0) {
q.push(mp[k][i]);
vis[mp[k][i]] = 1;
tmp.push_back(mp[k][i]);
cnt++;
}
}
}
return cnt;
}
int main()
{
ios::sync_with_stdio(false);
int n;
cin >> n;
mp.resize(n + 5);
string s;
cin >> s;
fill(vis, vis + MAXN, 0);
int len = s.length();
for(int i = 0; i < len; i++){
if(s[i] == 'B'){
// 黑色
vis[i + 1] = 1;
}
}
for(int i = 1; i < n; i++){
int a, b;
cin >> a >> b;
mp[a].push_back(b);
mp[b].push_back(a);
}
long long ans = 0;
for(int i = 1; i <= n; i++){
// 如果是黑色
if(vis[i] == 1) {
int now = 0;
int sum = 1; // 分两种情况, 1. 一个端点是黑色, 2. 两点都是白色
int vsize = mp[i].size();
for(int j = 0; j < vsize; j++){
// 该点是白色
if(vis[mp[i][j]] == 0) {
num[now++] = bfs(mp[i][j]);
sum += num[now - 1];
}
}
// 去除标记
while(!tmp.empty()) {
vis[tmp.back()] = 0;
tmp.pop_back();
}
// 统计
for(int j = 0; j < now; j++){
ans += num[j] * (sum - num[j]); // 1. num[j] * (sum - num[j] - 1) 2. num[j] * 1
sum = sum - num[j];
}
}
}
cout << ans << endl;
return 0;
}