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With respect to this problem : 2050G - Tree Destruction

Using vector of vector, compiled successfully and giving correct output

            int n;
            cin>>n;

            vector<vector<int>> g(n);
            vector<int> deg(n);
            for(int i=0, u, v; i<n-1; i++){
                cin>>u>>v;
                --u, --v;
                g[u].pb(v);
                g[v].pb(u);
                ++deg[u], ++deg[v];
            }

            int ans = 1;
            auto dfs = [&](auto self, int v, int p = -1) -> int {
                int mx1 = 0, mx2 = 0;
                for(auto &c : g[v]){
                    if(c == p) continue;
                    int res = self(self,c,v);
                    if(res > mx1) mx2 = mx1, mx1 = res;
                    else mx2 = max(mx2, res);
                }
                ans = max(ans, mx1+mx2-(mx1>0)-(mx2>0)+deg[v]);
                return (mx1-(mx1>0)+deg[v]-1);
            };

            dfs(dfs,0);
            cout<<ans<<nl;

But when trying the same with array of vectors, it doesn't compile

            int n;
            cin>>n;

            vector<int> g[n];
            vector<int> deg(n);
            for(int i=0, u, v; i<n-1; i++){
                cin>>u>>v;
                --u, --v;
                g[u].pb(v);
                g[v].pb(u);
                ++deg[u], ++deg[v];
            }

            int ans = 1;
            auto dfs = [&](auto self, int v, int p = -1) -> int {
                int mx1 = 0, mx2 = 0;
                for(auto &c : g[v]){
                    if(c == p) continue;
                    int res = self(self,c,v);
                    if(res > mx1) mx2 = mx1, mx1 = res;
                    else mx2 = max(mx2, res);
                }
                ans = max(ans, mx1+mx2-(mx1>0)-(mx2>0)+deg[v]);
                return (mx1-(mx1>0)+deg[v]-1);
            };

            dfs(dfs,0);
            cout<<ans<<nl;

Error by compiler -> error: use of deleted function 'main()::<lambda(auto:28, int, int)>::~<lambda>()' <lambda(auto:28, int, int)>::~<lambda>()' is implicitly deleted because the default definition would be ill-formed:

I want to know why this is happening. Can someone please tell me the reason behind this unknown behavior of lambda function ?

Comments (3)

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Intuition2.0

3 weeks ago, # |

Auto comment: topic has been updated by Intuition2.0 (previous revision, new revision, compare).

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unbit

An array of size n requires the n to be a compile-time constant while the vector is dynamically sized.

If you provide n at runtime with std::cin, the compiler does not know its value when constructing the array, leading to a compilation error.

This code 295344654 compiles fine, although TLEs because we are using global initialisation for a big graph for each test case, hence use a vector of vector for the graph in this situation 295345367

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