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IkramulMurad's blog

By IkramulMurad, history, 3 years ago, In English,

Hi, I was solving problem "20C — Dijkstra?". At first, I implemented through priority_queue. I got verdict "memory limit exceeded on test 31". I then realised that, if a single node relaxed more than once, priority_queue enqueue the node more than once. This may be the cause of memory limit exceed.

Then I have designed another algorithm with c++ std::set. I kept all the pair of cost and node number in that set. Then whenever a node to be relaxed, I firstly erase that node from the set, then update the node cost and insert to the set again. I believe this algorithm needs O(n) memory. Because there at most n pairs in the set. But I got verdict "memory limit exceeded on test 31" again. I don't know how it takes more memory or which test cases make it inefficient.

My implementation: http://ideone.com/2vNqOS

This is my first post. Thanks in advance. :)

 
 
 
 
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3 years ago, # |
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I think it is a compiler bug, I change the int to long long int for cost, and not type cast it, and it got AC.

See 24563357 for info

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    3 years ago, # ^ |
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    Thanks to Zun, I have found my fault. I wrote "int cost=pq.begin()->first;", where cost overflowed and became a negative value. That makes my Dijkstra function wrong.

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      3 years ago, # ^ |
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      right, it is not a compiler bug. but i am confused. How did your this 24561391 AC?

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        3 years ago, # ^ |
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        Because of using taken, less "+" operation has done and integer overflow hasn't occurred.