8. Bridge Edge in a Graph

The problem can be found at the following link: Question Link

Problem Description

Given an undirected graph with V vertices and E edges, along with a specific edge connecting vertices c and d, the task is to determine whether this edge is a bridge. An edge (c–d) is considered a bridge if removing it increases the number of connected components in the graph, meaning that vertices c and d were part of the same connected component before, but become disconnected after its removal.

Note: Sorry for uploading late, my Final Sem exam is going on.

Examples

Example 1:

Input:

• V = 5, E = 5

• Edges = {{0, 1}, {1, 2}, {2, 3}, {3, 4}, {2, 4}}

• c = 1, d = 2

Output:

• True

Explanation:

Removing the edge between nodes 1 and 2 disconnects the graph, indicating that it is a bridge.

Example 2:

Input:

• V = 5, E = 5

• Edges = {{0, 1}, {1, 2}, {2, 3}, {3, 4}, {2, 4}}

• c = 2, d = 4

Output:

• False

Explanation:

Removing the edge between nodes 2 and 4 does not affect the connectivity of the graph, indicating that it is not a bridge.

Constraints:

• $1 \leq V, E \leq 10^5$

• $0 \leq c, d \leq V-1$

My Approach

To determine if the edge (c, d) is a bridge, we can use the following approach:

1. Graph Representation: Represent the graph using an adjacency list for efficient traversal.

2. Depth First Search (DFS): Perform a DFS traversal to check the connectivity between nodes c and d after temporarily removing the edge (c, d).

3. Connectivity Check:

   • Temporarily remove the edge (c, d) from the graph.

   • Use DFS starting from node c to see if node d is still reachable.

   • If node d is not reachable from node c after removing the edge, then the edge (c, d) is a bridge.

Algorithm Steps:

1. Build the Adjacency List: Construct the adjacency list from the given list of edges.

2. Remove the Edge (c, d): Temporarily remove the edge by not including it in the adjacency list.

3. DFS Traversal:

   • Initialize a visited array to keep track of visited nodes.

   • Start DFS traversal from node c.

   • Mark all reachable nodes from c as visited.

4. Check Reachability of d:

   • After the DFS traversal, check if node d has been visited.

   • If node d is not visited, it means removing the edge (c, d) disconnects node d from node c, indicating that the edge is a bridge.

5. Restore the Edge (c, d): After the check, restore the edge to its original state in the adjacency list.

Time and Auxiliary Space Complexity

• Expected Time Complexity: O(V + E), as we perform a DFS traversal which visits each vertex and edge once.

• Expected Auxiliary Space Complexity: O(V), due to the space required for the visited array and the recursion stack during DFS.

Code (C++)

class Solution {
  public:
    void dfs(vector<vector<int>> &adj, int u, vector<bool> &vis) {
        vis[u] = true;
        for (int v : adj[u])
            if (!vis[v]) dfs(adj, v, vis);
    }

    bool isBridge(int V, vector<vector<int>> &edges, int c, int d) {
        vector<vector<int>> adj(V);
        for (auto &e : edges)
            if (!(e[0] == c && e[1] == d || e[0] == d && e[1] == c))
                adj[e[0]].push_back(e[1]), adj[e[1]].push_back(e[0]);
        vector<bool> vis(V);
        dfs(adj, c, vis);
        return !vis[d];
    }
};

⚡ Alternative Approaches

📊 2️⃣ Tarjan’s Algorithm for Bridge Detection

Algorithm Steps:

1. Convert edge list to adjacency list.

2. Perform DFS traversal and track discovery time tin[u] and lowest reachable time low[u].

3. For every node, update the low value based on its descendants.

4. An edge (u, v) is a bridge if low[v] > tin[u].

class Solution {
    int timer;
    bool dfs(int u, int p, vector<int> adj[], vector<int> &tin, vector<int> &low, int c, int d) {
        tin[u] = low[u] = ++timer;
        for (int v : adj[u]) {
            if (v == p) continue;
            if (!tin[v]) {
                if (dfs(v, u, adj, tin, low, c, d)) return true;
                low[u] = min(low[u], low[v]);
                if ((u == c && v == d || u == d && v == c) && low[v] > tin[u]) return true;
            } else low[u] = min(low[u], tin[v]);
        }
        return false;
    }
public:
    bool isBridge(int V, vector<vector<int>> &edges, int c, int d) {
        vector<int> adj[V], tin(V), low(V);
        for (auto &e : edges) adj[e[0]].push_back(e[1]), adj[e[1]].push_back(e[0]);
        timer = 0;
        for (int i = 0; i < V; i++)
            if (!tin[i] && dfs(i, -1, adj, tin, low, c, d))
                return true;
        return false;
    }
};

📝 Complexity Analysis:

• Time Complexity: O(V + E)

• Space Complexity: O(V + E) (Adjacency list + recursion stack)

✅ Why This Approach?

Efficient for detecting all bridges in a graph in one pass. The fastest method if this is done repeatedly or on large graphs.

📊 3️⃣ Remove Edge + DFS Reachability Check (Iterative)

Algorithm Steps:

1. Build an adjacency list excluding the edge (c, d).

2. Perform an iterative DFS using a stack starting from node c.

3. If d is not reachable from c, then (c, d) is a bridge.

class Solution {
public:
    bool isBridge(int V, vector<vector<int>> &edges, int c, int d) {
        vector<vector<int>> adj(V);
        for (auto &e : edges)
            if (!(e[0] == c && e[1] == d || e[0] == d && e[1] == c))
                adj[e[0]].push_back(e[1]), adj[e[1]].push_back(e[0]);
        vector<bool> vis(V, false);
        stack<int> st;
        st.push(c);
        while (!st.empty()) {
            int u = st.top(); st.pop();
            if (vis[u]) continue;
            vis[u] = true;
            for (int v : adj[u]) if (!vis[v]) st.push(v);
        }
        return !vis[d];
    }
};

📝 Complexity Analysis:

• Time Complexity: O(V + E)

• Space Complexity: O(V + E) (Adjacency list + visited + stack)

✅ Why This Approach?

Simple to implement and avoids recursion limits; best when only one edge needs checking, not all bridges.

🆚 Comparison of Approaches

Approach	⏱️ Time Complexity	🗂️ Space Complexity	✅ Pros	⚠️ Cons
Tarjan’s Algorithm	🟢 O(V + E)	🟡 O(V + E)	Fastest for multiple bridge queries	More complex; recursive
Remove Edge + DFS (Recursive)	🟢 O(V + E)	🟢 O(V + E)	Intuitive; easier to write	Recursion depth limits
Remove Edge + DFS (Iterative)	🟢 O(V + E)	🟡 O(V + E)	Stack-based; avoids recursion	Slightly more verbose

✅ Best Choice?

• Use Tarjan’s when multiple bridge checks or global bridge detection is needed.

• Use DFS-based methods for one-time or simpler cases.

Code (Java)

class Solution {
    void dfs(List<List<Integer>> g, boolean[] vis, int u) {
        vis[u] = true;
        for (int v : g.get(u)) if (!vis[v]) dfs(g, vis, v);
    }

    public boolean isBridge(int V, int[][] edges, int c, int d) {
        List<List<Integer>> g = new ArrayList<>();
        for (int i = 0; i < V; i++) g.add(new ArrayList<>());
        for (int[] e : edges)
            if (!(e[0] == c && e[1] == d || e[0] == d && e[1] == c)) {
                g.get(e[0]).add(e[1]);
                g.get(e[1]).add(e[0]);
            }
        boolean[] vis = new boolean[V];
        dfs(g, vis, c);
        return !vis[d];
    }
}

Code (Python)

class Solution:
    def dfs(self, g, vis, u):
        vis[u] = 1
        for v in g[u]:
            if not vis[v]: self.dfs(g, vis, v)

    def isBridge(self, V, edges, c, d):
        g = [[] for _ in range(V)]
        for u, v in edges:
            if (u, v) != (c, d) and (v, u) != (c, d):
                g[u].append(v)
                g[v].append(u)
        vis = [0] * V
        self.dfs(g, vis, c)
        return not vis[d]

Contribution and Support:

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