Ishan.nitj's blog

Time complexity of std::distance
By Ishan.nitj, history, 8 years ago, In English

I am trying to solve this problem http://codeforces.com/contest/652/problem/D. My approach is http://codeforces.com/contest/652/submission/18906153 which is nlogn and should pass the time constraint.However I am getting TLE. I think this might be due std::distance which i have used to calculate distance b/w 2 elements in set.Please HELP.

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8 years ago, # |
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For random access iterators, std::distance is O(1). Unfortunately set iterator does not support random access, so the std::distance algorithm has to iterate over the pointers to compute distance, and worst case is O(n). You can achieve O(1) distance calc in sets if you use order-statistic balanced BST (there is some post in Codeforces teaching how to build this data structure using some hidden C++ libraries)

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    8 years ago, # ^ |
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    Can you provide the link to it? Also whats the time complexity with std::vector

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      8 years ago, # ^ |
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      Click.

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      8 years ago, # ^ |
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      For std::vector complexity is O(1) since the iterators support random access. You can also use subtraction operator with the same effect.

      Example : (int) (lower_bound(V.begin(),V.end(),X) — V.begin()) returns distance from begin to first element in array with value >= X, so it's the same as std::distance(V.begin(), lower_bound(V.begin(),V.end(),X))

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    8 years ago, # ^ |
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    Thnks. Got AC using it. http://codeforces.com/contest/652/submission/18907155

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