59 lines
1.7 KiB
C++
59 lines
1.7 KiB
C++
#pragma once
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// https://en.wikipedia.org/wiki/Median_of_medians
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int findMedian(std::vector<size_t> values) {
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size_t median;
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size_t size = values.size();
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median = values[(size / 2)];
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return median;
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}
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int findMedianOfMedians(std::vector<std::vector<size_t> > values) {
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std::vector<size_t> medians;
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for (size_t i = 0; i < values.size(); i++) {
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size_t m = findMedian(values[i]);
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medians.push_back(m);
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}
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return findMedian(medians);
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}
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size_t getMedianOfMedians(const std::vector<size_t> values, size_t k) {
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// Divide the list into n/5 lists of 5 elements each
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std::vector<std::vector<size_t> > vec2D;
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size_t count = 0;
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while (count != values.size()) {
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size_t countRow = 0;
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std::vector<size_t> row;
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while ((countRow < 5) && (count < values.size())) {
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row.push_back(values[count]);
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count++;
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countRow++;
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}
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vec2D.push_back(row);
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}
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// Calculating a new pivot for making splits
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size_t m = findMedianOfMedians(vec2D);
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// Partition the list into unique elements larger than 'm' (call this sublist L1) and those smaller them 'm' (call this sublist L2)
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std::vector<size_t> L1, L2;
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for (size_t i = 0; i < vec2D.size(); i++) {
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for (size_t j = 0; j < vec2D[i].size(); j++) {
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if (vec2D[i][j] > m) {
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L1.push_back(vec2D[i][j]);
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}
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else if (vec2D[i][j] < m) {
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L2.push_back(vec2D[i][j]);
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}
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}
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}
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if (k <= L1.size()) {
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return getMedianOfMedians(L1, k);
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}
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else if (k > (L1.size() + 1)) {
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return getMedianOfMedians(L2, k - ((int)L1.size()) - 1);
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}
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return m;
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} |