C ++程序查找序列中的第k个最大元素
在此程序中，我们需要从序列中提取第K个最大元素。通过使用最大堆来解决该问题，可以提高该技术的时间复杂度。该程序的时间复杂度为O（n+k*log(n)）。
算法
Begin Send the max of the heap to the end of the sequence. Heapify the remaining sequence. Repeat the process for ‘k’ times. Print the final state of the array. Print the max from the heap extracted from kth iteration as a result. End.
范例程式码
#include <iostream>
using namespace std;
void MaxHeapify(int a[], int i, int n) {
int j, t;
t = a[i];
j = 2*i;
while (j <= n) {
if (j < n && a[j+1] > a[j])
j = j+1;
if (t > a[j])
break;
else if (t <= a[j]) {
a[j/2] = a[j];
j = 2*j;
}
}
a[j/2] = t;
return;
}
void Build_MaxHeapify(int a[], int n) {
int i;
for(i = n/2; i >= 1; i--)
MaxHeapify(a, i, n);
}
int main() {
int n, i, temp, k;
cout<<"\nEnter the number of data element to be sorted: ";
cin>>n;
n++;
int arr[n];
for(i = 1; i < n; i++) {
cout<<"Enter element "<<i<<": ";
cin>>arr[i];
}
cout<<"\nEnter the k value: ";
cin>>k;
Build_MaxHeapify(arr, n-1);
for(i = n-1; i >= n-k; i--) {
temp = arr[i];
arr[i] = arr[1];
arr[1] = temp;
MaxHeapify(arr, 1, i - 1);
}
cout<<"\nAfter max-heapify the given array "<<k<<" times the array state is: ";
for(i = 1; i < n; i++)
cout<<"->"<<arr[i];
cout<<"\n\nThe "<<k<<"th largest element is: "<<arr[n-k];
return 0;
}输出结果
Enter the number of data element to be sorted: 5 Enter element 1: 20 Enter element 2: 10 Enter element 3: 30 Enter element 4: 70 Enter element 5: 60 Enter the k value: 2 After max-heapify the given array 2 times the array state is: ->30->20->10->60->70 The 2th largest element is: 60
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