56 lines
1.4 KiB
C++
56 lines
1.4 KiB
C++
// Algorithm Type: Divide & Conquer
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// Time Complexity: O(n*log(n))
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#include <iostream>
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#include <iterator>
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#include <fstream>
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#include <vector>
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using namespace std;
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long long int count_split_inv(vector<int> &vec, vector<int> &left, vector<int> &right) {
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long long int split_inv = 0;
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int ridx = 0, lidx = 0;
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int size = vec.size();
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int lsize = left.size();
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int rsize = right.size();
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for (int i = 0; i < size; i++) {
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if (lidx != lsize && ridx != rsize) {
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if (right[ridx] <= left[lidx]) {
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vec[i] = right[ridx];
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ridx++;
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split_inv += lsize - lidx;
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} else {
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vec[i] = left[lidx];
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lidx++;
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}
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} else if (lidx == lsize) {
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vec[i] = right[ridx];
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ridx++;
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} else if (ridx == rsize) {
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vec[i] = left[lidx];
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lidx++;
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}
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}
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return split_inv;
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}
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long long int count_inversions(vector<int> &vec) {
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int size = vec.size();
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if (size == 1) {
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return 0;
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}
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vector<int> left(vec.begin(), vec.begin() + size / 2);
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vector<int> right(vec.begin() + size / 2, vec.end());
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long long int left_inv = count_inversions(left);
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long long int right_inv = count_inversions(right);
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long long int split_inv = count_split_inv(vec, left, right);
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return left_inv + right_inv + split_inv;
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}
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int main() {
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vector<int> v{8, 2, 1, 5, 7, 3, 9, 2, 0, 1};
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cout << count_inversions(v) << endl;
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return 0;
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}
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