|
| 1 | +#include<iostream> |
| 2 | +using namespace std; |
| 3 | + |
| 4 | + |
| 5 | +void maxHeapify(int*, int, int); |
| 6 | +void buildHeap(int*, int); |
| 7 | +void heapsort(int*, int); |
| 8 | + |
| 9 | +int main() { |
| 10 | + |
| 11 | + int arr[] = { 4,2,3,1,5,6,8,}; |
| 12 | + |
| 13 | + heapsort(arr, sizeof(arr) / sizeof(int)); |
| 14 | + |
| 15 | + for (int i = 0; i < sizeof(arr) / sizeof(int); i++) { |
| 16 | + cout << arr[i] << " "; |
| 17 | + } |
| 18 | + |
| 19 | + return 0; |
| 20 | +} |
| 21 | + |
| 22 | +void maxHeapify(int* arr, int n, int i) { |
| 23 | + if (i > n / 2 - 1) { |
| 24 | + //if index of element is greater than i/2 it is in the last level of the heap. no need to call maxHeapify for it. |
| 25 | + return; |
| 26 | + } |
| 27 | + if (arr[i] > arr[2 * i] && arr[i] > arr[(2 * i) + 1]) { |
| 28 | + //heap property already satisfied. |
| 29 | + return; |
| 30 | + } |
| 31 | + |
| 32 | + int maxInx = i, temp; |
| 33 | + |
| 34 | + while (i <= (n / 2 - 1)) { |
| 35 | + //finding the index of the largest child of i. |
| 36 | + if (arr[2 * i + 1] > arr[i] && (2 * i + 1) < n) { |
| 37 | + maxInx = 2 * i + 1; |
| 38 | + } |
| 39 | + if (arr[(2 * i) + 2] > arr[maxInx] && (2 * i + 2) < n) { |
| 40 | + maxInx = (2 * i) + 2; |
| 41 | + } |
| 42 | + //if no change occurs. then return. |
| 43 | + if (i == maxInx) { return; } |
| 44 | + |
| 45 | + //having obtained the index of the largest node swap it with i. |
| 46 | + //swapping arr[i] with arr[maxInx] |
| 47 | + temp = arr[maxInx]; |
| 48 | + arr[maxInx] = arr[i]; |
| 49 | + arr[i] = temp; |
| 50 | + i = maxInx; |
| 51 | + } |
| 52 | + |
| 53 | +} |
| 54 | + |
| 55 | +void buildHeap(int* arr, int n) { |
| 56 | + //start calling from the last parent in the heap(n/2) and go upto the root node. |
| 57 | + //start from the last parent instead of root because here the loop invacriant is that heap condition is |
| 58 | + //satisfied for all nodes other than the one for which it is called. which would not be true when it is |
| 59 | + //called for the root node of the array when the array is compeletely random. |
| 60 | + //refer to CORMEN for more details. |
| 61 | + for (int i = (n / 2 - 1); i >= 0; i--) { |
| 62 | + maxHeapify(arr, n, i); |
| 63 | + } |
| 64 | +} |
| 65 | + |
| 66 | +void heapsort(int* arr, int n) { |
| 67 | + buildHeap(arr, n); |
| 68 | + |
| 69 | + int temp; |
| 70 | + while (n > 0) { |
| 71 | + //after the heap is built take the element a[0], swap it with the last element, discard it and call maxheapify |
| 72 | + //for the root element |
| 73 | + |
| 74 | + temp = arr[n - 1]; |
| 75 | + arr[n - 1] = arr[0]; |
| 76 | + arr[0] = temp; |
| 77 | + n--; |
| 78 | + maxHeapify(arr, n, 0); |
| 79 | + } |
| 80 | +} |
0 commit comments