Need help! I tried to solve SPOJ QTREE4. I used centroid decomposition, but still getting TLE (time limit exceeded). In my code, a query is processed in O(log(n)^2). How can I improve complexity? Any suggestions. Here is my code.
You may try a segment tree on the euler tour.You need to understand the euler tour/rmq algorithm for lca first (it can be found here https://en.wikipedia.org/wiki/Lowest_common_ancestor ).Then,build a segment tree holding the maximum value of lev[node1]-2*lev[node2]+lev[node3] where node1 and node3 are at first occurrence,both are white and node2 can be found between them(and is not neccesarily white)- lev[x] denotes the level of node x.Of course,you need to maintain some helping values (e.g maximum of lev[node1]-2*lev[node2]) but I find this a good segment tree exercise.Finally, you get O(logn) per update.
This function 'merges' 2 segment nodes(you now the answers for [a;b] in B,for[b+1;c] in C and you get them for [a,c] in A.I'm sorry it isn't very explicit, but i'll try to explain which is every part of the structure.The rest of the segment tree and building euler tour is more of a prequisite and I would PM if requested, but I find the most important part here.
Code
void merg(node &A,node B,node C)
{
A.c[0]=max(B.c[0],C.c[0]); //maximum level of a node
A.c[1]=max(B.c[1],C.c[1]); //maximum of -2*level of a node (the lca we try to find)
A.c[2]=max(B.c[0]+C.c[1],max(B.c[2],C.c[2])); //a partial maximum used to get the final answer
A.c[3]=max(B.c[1]+C.c[0],max(B.c[3],C.c[3])); //another partial maximum
A.c[4]=max(max(B.c[0]+C.c[3],B.c[2]+C.c[0]),max(B.c[4],C.c[4])); //the maximum distance
}
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You may try a segment tree on the euler tour.You need to understand the euler tour/rmq algorithm for lca first (it can be found here https://en.wikipedia.org/wiki/Lowest_common_ancestor ).Then,build a segment tree holding the maximum value of
lev[node1]-2*lev[node2]+lev[node3]where node1 and node3 are at first occurrence,both are white and node2 can be found between them(and is not neccesarily white)-lev[x]denotes the level of node x.Of course,you need to maintain some helping values (e.g maximum oflev[node1]-2*lev[node2]) but I find this a good segment tree exercise.Finally, you get O(logn) per update.Can you please Elaborate on how to Maintain the Structure. A submission Code would be Extremely Helpful.
This function 'merges' 2 segment nodes(you now the answers for
[a;b]inB,for[b+1;c]inCand you get them for[a,c]inA.I'm sorry it isn't very explicit, but i'll try to explain which is every part of the structure.The rest of the segment tree and building euler tour is more of a prequisite and I would PM if requested, but I find the most important part here.