108.冗余连接
注意init初始化
改进：
其实只有一条边冗余，改为，如果两条边在同一个集合里，就输出，不然加入。
#include <iostream>
#include <vector>
using namespace std;
int n = 1005;
vector<int> father(n,0);
void init(){
for (int i=0;i<n;i++){
father[i] = i;
}
}
int find(int u){
return u == father[u]? u: father[u] = find(father[u]);
}
bool isSame(int u, int v){
u = find(u);
v = find(v);
return u == v;
}
void join(int u, int v){
u = find(u);
v = find(v);
if(u==v){
return;
}
father[u] = v;
}
int main(){
int N;
cin >>N;
init(); //1111
int s,t,redun_s, redun_t;
bool result;
while(N--){
cin>>s>>t;
result = isSame(s,t);
if (result){
redun_s = s;
redun_t = t;
}
join(s,t);
}
cout << redun_s <<" "<< redun_t << endl;
return 0;
}
109.冗余连接II
不好做：
1.入度为2：看删哪条边可以形成树，而不是环（因为只有两种可能，一个树，一个环）
2.没有入度为2的点：有向环，删最后形成环的那条
#include <iostream>
#include <vector>
using namespace std;
int n;
vector<int> father(1001, 0);
void init() {
for (int i = 0; i < n; i++) {
father[i] = i;
}
}
int find(int u) {
return u == father[u] ? u : father[u] = find(father[u]);
}
bool isSame(int u, int v) {
u = find(u);
v = find(v);
return u == v;
}
void join(int u, int v) {
u = find(u);
v = find(v);
if (u == v) {
return;
}
father[v] = u;
}
bool isTreeAfterRemoveVec(const vector<vector<int>>& edges, int v) {
init();
for (int i = 0; i < n; i++) {
if (i == v) {
continue;
}
if (isSame(edges[i][0], edges[i][1])) {
return false;
}
join(edges[i][0], edges[i][1]);
}
return true;
}
void getRemoveEdge(const vector<vector<int>>& edges) {
init();
for (int i = 0; i < n; i++) {
if (isSame(edges[i][0], edges[i][1])) {
cout << edges[i][0] << " " << edges[i][1];
return;
}
join(edges[i][0], edges[i][1]);
}
}
int main() {
// int N;
cin >> n;
// init(); //1111
// n = 3;
// vector<vector<int>> grid;
// grid = {
// {1,2},
// {1,3},
// {2,3}
// };
vector<vector<int>> edges;
vector<int> degrees(n+1, 0);
int s, t;
for(int i=0;i<n;i++) {
cin >> s >> t;
// s = grid[i][0];
// t = grid[i][1];
edges.push_back({ s,t });
degrees[t]++;
}
//计算入度
vector<int> vec;
for (int i = 0; i < n; i++) {
//cout << degrees[edges[i][1]] << " ";
if (degrees[edges[i][1]] == 2) {
vec.push_back(i);
}
}
//情况1：入度为2，看删哪个
if (vec.size() > 1) {
if (isTreeAfterRemoveVec(edges, vec[1])) {
cout << edges[vec[1]][0] << " " << edges[vec[1]][1] << endl;
}
else {
cout << edges[vec[0]][0] << " " << edges[vec[0]][1] << endl;
}
return 0;
}
//情况2：有向环
getRemoveEdge(edges);
return 0;
}