Document LightOJ 1101 – A Secret Mission

Added problem summary, key observation, strategy, complexity analysis, and implementation details for LightOJ 1101.

Author: sadat2103108

1101/en.md

@@ -1 +1,236 @@
+# LightOJ 1101 – A Secret Mission
 
+## Problem Summary
+
+We are given a connected weighted graph with (N) cities and (M) roads.
+Each road has a *danger value*.
+
+For each query (s, t), we must find a path from (s) to (t) such that:
+
+> the maximum danger of any road on the path is minimized.
+
+This is known as a **minimax path problem**.
+
+---
+
+## Key Observation
+
+If we build a **Minimum Spanning Tree (MST)** of the graph, then:
+
+> For any two nodes, the path between them in the MST minimizes the maximum edge weight among all possible paths in the original graph.
+
+This is a well-known property of MSTs.
+Therefore, once we construct the MST, we only need to answer queries on that tree.
+
+---
+
+## Strategy
+
+1. Build the MST using **Prim’s algorithm**
+2. Convert the graph into a tree
+3. Preprocess the tree using **Binary Lifting**
+4. Store:
+
+   * `anc[u][i]` → (2^i)-th ancestor of node (u)
+   * `mx[u][i]` → maximum edge weight from (u) to its (2^i)-th ancestor
+5. For each query:
+
+   * compute LCA of (u) and (v)
+   * find maximum edge on path (u to LCA) and (v to LCA) using binary lifting
+   * answer = max of the two above values.
+
+---
+
+## Complexity
+
+* MST construction: (O(M log N))
+* Binary lifting preprocessing: (O(N log N))
+* Each query: (O(log N))
+
+This easily fits within constraints.
+
+---
+
+## Implementation
+
+```cpp
+#include<bits/stdc++.h>
+using namespace std;
+#define br cout<<"\n";
+#define ll long long
+#define loop(n) for(int i=0; i<(n); i++)
+#define fr(i,init, n) for(int i=(init); i<(n); i++)
+#define revl(i,init) for(int i=(init-1); i>=0; i--)
+#define pb push_back
+#define all(v) v.begin(),v.end()
+#define nl "\n"
+
+
+const int N =  50005, LOG=18 ;
+vector<pair<int,int>> adj2[N], adj[N];
+bool vis[N];
+int anc[N][LOG], mx[N][LOG], dep[N];
+
+
+// clear global variables for each test case
+void clear(){
+    loop(N){
+        vis[i] = false;
+        adj2[i].clear();
+        adj[i].clear();
+    }
+    memset(anc,0, sizeof(anc));
+    memset(mx, 0, sizeof(mx));
+    memset(dep, 0, sizeof(dep));
+}
+
+// create the MST from adj2[] and build the new one in adj[]
+void createMST(){
+    
+    priority_queue<
+        tuple<int,int,int>,
+        vector<tuple<int,int,int>>,
+        greater<tuple<int,int,int>>
+    > pq;
+    
+    pq.push({-1, 1,0});
+    
+    while(! pq.empty()){
+        int wt, node, parent;
+        tie(wt, node, parent) = pq.top();
+        pq.pop();
+        
+        if(vis[node]) continue;
+        
+        vis[node] = true;
+        
+        if(parent!=0){
+            adj[parent].pb({node,wt});
+            adj[node].pb({parent,wt});
+        }
+        
+        
+        for(auto child: adj2[node]){
+            if(!vis[child.first]){
+                pq.push({child.second, child.first, node});
+            }
+        }
+    }
+        
+}
+    
+// dfs for binary-lifting precomputations
+void dfs(int node, int par=0, int wt=0){
+
+    dep[node] = dep[par]+1;
+    anc[node][0] = par;
+
+    mx[node][0]= wt;
+
+    fr(i,1,LOG){
+        anc[node][i] = anc[ anc[node][i-1] ][i-1];
+        mx[node][i] = max( mx[ anc[node][i-1] ][i-1] , mx[node][i-1] );
+    }
+
+    for(auto child: adj[node]){
+        if(child.first != par){
+            dfs(child.first, node, child.second);
+        }
+    }
+}
+
+int lca(int u, int v){
+    if(dep[u]< dep[v]) swap(u,v);
+
+    revl(i,LOG){
+        if( dep[ anc[u][i] ] >= dep[v] ){
+            u = anc[u][i];
+        }
+    }
+
+    if(u==v) return u;
+
+    revl(i,LOG){
+        if(anc[u][i] != anc[v][i]){
+            u = anc[u][i];
+            v = anc[v][i];
+        }
+    }
+
+    return anc[u][0];
+
+}
+
+// maximum edge between a node and its ancestor
+int maxedge(int node, int l){
+    int dis = dep[node] - dep[l];
+
+    int ans = 0;
+
+    revl(i,LOG){
+        if(dis & (1<<i)){
+            ans = max( ans, mx[node][i]   );
+            node= anc[node][i];
+        }
+    }
+
+    return ans;
+
+}
+
+int query(int u, int v){
+
+    int l = lca(u,v);
+
+    return max( maxedge(u,l), maxedge(v,l) );
+
+}
+
+
+
+int tc = 1;
+void SOLVE(){
+    clear();
+    cout<<"Case "<<tc++<<":"<<nl;
+
+    int n,m; cin>>n>>m;
+    while(m--){
+        int a,b,c; cin>>a>>b>>c;
+        adj2[a].pb({b,c});
+        adj2[b].pb({a,c});
+    }
+
+    createMST();
+
+    dfs(1);
+
+    int q; cin>>q;
+    while(q--){
+        int u,v; cin>>u>>v;
+        cout<<query(u,v)<<nl;
+    }
+
+}   
+
+signed main(){
+    ios_base::sync_with_stdio(0);
+    cin.tie(0);
+    int t=1;
+
+    cin>>t;///////////////////// 
+
+    while(t--) SOLVE();
+    return 0;
+}
+```
+
+---
+
+## References
+
+* https://cp-algorithms.com/graph/mst_prim.html
+* https://cp-algorithms.com/graph/lca_binary_lifting.html
+
+---
+
+**Author:** Sadat