59 lines
1.4 KiB
C++
59 lines
1.4 KiB
C++
//
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// Created by Matthew Gretton-Dann on 07/12/2021.
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//
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#include <iostream>
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#include <list>
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#include <string>
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/* Consider the case of 8 elves:
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* Start: 1 2 3 4 5 6 7 8
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* 1 -> 5: 1 2 3 4 6 7 8
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* 2 -> 6: 1 2 3 4 7 8
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* 3 -> 8: 1 2 3 4 7
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* 4 -> 1: 2 3 4 7
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* 7 -> 3: 2 4 7
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* 2 -> 4: 2 7
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* 7 -> 2: 7
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*
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* Looking at this we can see that we start by removing the element half-way round the list. The
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* next element removed is always either the one after the one just removed, or the one just beyond
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* that.
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*
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* Which one is removed depends on whether there are an even or odd number of elements left.
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*
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* We make use of this to realize we don't have to track the current elf who is getting parcels
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* only the elf that is having parcels stolen from them.
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*/
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auto main() -> int
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{
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std::string line;
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if (!std::getline(std::cin, line)) {
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std::cerr << "Unable to read input.\n";
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return 1;
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}
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auto n{std::stoul(line)};
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std::list<unsigned long> elves;
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for (unsigned long i{1}; i <= n; ++i) {
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elves.push_back(i);
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}
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/* it points to the first elf to remove. */
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auto it{elves.begin()};
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for (unsigned long i{0}; i < n / 2; ++i) {
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++it;
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}
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while (n > 1) {
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it = elves.erase(it);
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if (it == elves.end()) {
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it = elves.begin();
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}
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if ((--n) % 2 == 0) {
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++it;
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}
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if (it == elves.end()) {
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it = elves.begin();
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}
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}
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std::cout << "Last elf: " << elves.front() << '\n';
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} |