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find-median-in-linear-time.cpp
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find-median-in-linear-time.cpp
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#include <algorithm>
#include <array>
#include <iostream>
#include <iterator>
#include <random>
using namespace std;
void swap(int *a, int *b) {
int *temp = a;
*a = *b;
*b = *temp;
}
int randomRange(int l, int r) { return l + (std::rand() % (r - l + 1)); }
int partition(int *arr, int l, int r, int k) {
int i;
for (i = l; i < r; i++)
if (arr[i] == k)
break;
swap(arr[i], arr[r]);
i = l;
for (int j = l; j <= r - 1; j++) {
if (arr[j] <= k) {
swap(arr[i], arr[j]);
i++;
}
}
swap(arr[i], arr[r]);
return i;
}
int selectMedian(int *arr, int l, int r) {
int k = (r - l + 1) / 2;
int p = partition(
arr, l, r,
arr[k]); // use median of median for better peroformance in worst case
if (p - l == k - 1)
return arr[p];
else if (p - l > k)
return selectMedian(arr, l, p - 1);
return selectMedian(arr, p + 1, r);
}
int main(int argc, char const *argv[]) {
int arr[] = {12, 3, 5, 17, 4, 19, 26};
const int n = sizeof(arr) / sizeof(arr[0]);
array<int, n> arr1;
std::copy(std::begin(arr), std::end(arr), std::begin(arr1));
std::cout << selectMedian(arr, 0, n) << std::endl;
sort(arr1.begin(), arr1.end());
for (int i = 0; i < n; i++)
{
cout << arr1.at(i) << " ";
}
cout << endl;
std::cout << arr1[n/2] << std::endl;
return 0;
}