解題區/解題區 - UVa

UVa 12783 - Weak Links

contents

  1. 1. Problem
  2. 2. Sample Input
  3. 3. Sample Output
  4. 4. Solution

Problem

是否存在 weak link，使得兩個 component 會因為 weak link 斷掉而無法連通。

Sample Input

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4 3
0 3
0 1
0 2
7 8
0 1
0 2
1 3
2 3
3 4
4 5
4 6
5 6
0 0

Sample Output

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3 0 1 0 2 0 3
1 3 4

Solution

找橋 bridge！利用 back edge 的深度進行判斷。

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#include <stdio.h>
#include <string.h>
#include <vector>
#include <algorithm>
using namespace std;
vector<int> g[1024];
int visited[1024], depth[1024];
vector< pair<int, int> > bridge;
int findBridge(int u, int p, int dep) {
    visited[u] = 1, depth[u] = dep;
    int back = 0x3f3f3f3f;
    for(int i = 0; i < g[u].size(); i++) {
        int v = g[u][i];
        if(v == p)	continue;
        if(!visited[v]) {
            int b = findBridge(v, u, dep+1);
            if(b > dep)
                bridge.push_back(make_pair(u, v));
            back = min(back, b);
        } else {
            back = min(back, depth[v]);
        }
    }
    return back;
}
int main() {
    int n, m, q, x, y;
    while(scanf("%d %d", &n, &m) == 2 && n+m) {
        for(int i = 0; i < n; i++)
            g[i].clear();
        for(int i = 0; i < m; i++) {
            scanf("%d %d", &x, &y);
            g[x].push_back(y);
            g[y].push_back(x);
        }
        
        bridge.clear();
        memset(visited, 0, sizeof(visited));
        for(int i = 0; i < n; i++) {
            if(!visited[i]) {
                findBridge(i, -1, 0);
            }
        }
        for (int i = 0; i < bridge.size(); i++)
            if (bridge[i].first > bridge[i].second)
                swap(bridge[i].first, bridge[i].second);
        sort(bridge.begin(), bridge.end());
        printf("%d", bridge.size());
        for (int i = 0; i < bridge.size(); i++)
            printf(" %d %d", bridge[i].first, bridge[i].second);
        puts("");
    }
    return 0;
}