How can I implement it O(n)?

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Автор Tintin, 14 лет назад, По-английски

Your text to link here... My dp solution is O(n^2) but here n=50000 and time is 1 sec. Is there any mathematical formula which reduces time within 1 sec. Any hints...Thanks

dp problem

Tintin
14 лет назад
6

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yeputons

14 лет назад, скрыть # |

Try this idea: calculate sums in all prefixes (from empty to the whole sequence) and then sum of elements in a some substring is difference of two numbers. Hope this helps. If not, I can give you detailed solution.

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Tintin

14 лет назад, скрыть # ^ |

I don't get it properly.

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yeputons

14 лет назад, скрыть # ^ |

Look: you have an array: 1 2 3 4 5. It has the following prefixes: "", "1", "1 2", and so on. You calculate sum of numbers in each such prefix and get 0, 1, 1+2=3, 1+2+3=6, 10 and 15 (let's call it s[]). Then, to calculate, for example, sum from 2 to 4, you just calculate s[4] - s[1]=9. Elements before 2 were reduced.

This idea can help you solving this problem: you run down left border of a substring and then you can calc amount of required substrings in O(1) for each left border.

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Tintin

14 лет назад, скрыть # ^ |

I get your first ideal and I use it in my code.But still don't get your second idea. here is my code:http://pastebin.com/abRxS9kg

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goo.gl_SsAhv

14 лет назад, скрыть # ^ |

← Rev. 2 →

s[0] = 0;
for (int i = 0; i < a.size(); i++)
  s[i + 1] += s[i] + a[i];
for (int i = 0; i <= s.size(); i++) {
  res += cnt[s[i] % d];
  cnt[s[i] % d]++;
}

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