. # MinHeap Implementation step-by-step.
As a first step, we define an array based MinHeap, a constructor that takes an array passed as argument to initialize the heap and a print method that outputs the contents of the heap. We override toString to stringify the object and a getter method to get the value at a specific index.
// MinHeap.java
public class MinHeap {
int[] arr;
/** Initializes the array object */
public MinHeap(int[] keys) {
arr = keys; // Make arr point to keys array
}
/** An utility method to print contents of MinHeap object */
public void print() {
System.out.print(arr[0]);
for (int i=1; i<arr.length; i++)
System.out.print(" " + arr[i]);
System.out.println();
}
/** This overriden function is used to stringfy the contents of the object */
@override
public String toString() {
StringBuffer sb = new StringBuffer();
sb.append(arr[0]);
for (int i=1; i< arr.length; i++)
sb.append(" " + arr[i]);
return sb.toString();
}
/** Retrieve the value at index i */
public int get(int i) {
return arr[i];
}
}// MinHeapInitTest.java
public class MinHeapInitTest {
public static void main(String[] args) {
int[] keys = {5, 3, 8, 6, 2, 1, 7, 9, 4, 0};
MinHeap m = new MinHeap(keys);
m.print(); // prints the above array
System.out.println(m); // does same job as m.print();
// automatically invokes toString of m
System.out.println( m.get(3) ); // prints 6 - data at index 3
}
}Implement methods to compute the indices of parent, left child and right child, given an index i. With 0-based indexing,
- parent of node at index i is located at index (i-1)/2.
- left child of node at index i is located at index (2i+1).
- right child of node at index i is located at index (2i+2).
Corner cases:
- root (located at index 0) does not have parent
- leaf (located at indices greater (n-1)/2 where n is the number of nodes) does not have both left and right children
- The internal node at index (n-1)/2 may have left child only if n is even.
// MinHeap.java
public class MinHeap {
int[] arr;
public MinHeap(int[] keys) { ... }
public void print() { ... }
public String toString() { ... }
public int get(int i) { ... }
/** Given index i, retrieve the index of its parent */
public int parent(int i) {
// Two cases to handle - root and non-root cases
if (i == 0) // root node
return 0;
else
return (i-1)/2;
/* Note: if i==0, (i-1)/2 = -1/2 = 0.
So last statement alone is enough */
}
public int left(int i) {
// Two cases to handle - internal and leaf nodes
if ( 2*i+1 < arr.length ) // 1. internal node
return 2*i+1;
else // 2. leaf node
return -1;
}
public int right(int i) {
// Two cases to handle - internal and leaf nodes
if ( 2*i+2 < arr.length ) // 1. internal node
return 2*i+2;
else // 2. leaf node
return -1;
}
}// MinHeapTest.java
public class MinHeapTest {
public static void main(String[] args) {
int[] keys = {5, 3, 8, 6, 2, 1, 7, 9, 4, 0};
MinHeap m = new MinHeap(keys);
System.out.println( m.parent(0) ); // prints 0
System.out.println( m.get(m.parent(0)) ); // prints 5
System.out.println( m.parent(8) ); // prints 3
System.out.println( m.get(m.parent(5)) ); // prints 6
if (m.left(3) != -1) // print only if left child exists
System.out.println( m.get(m.left(3)) ); // prints 9
if (m.left(5) != -1) // print only if left child exists
System.out.println( m.get(m.left(5) ); // unreachable
if (m.right(3) != -1) // print only if right child exists
System.out.println( m.get(m.right(3)) ); // prints 4
if (m.right(5) != -1) // print only if right child exists
System.out.println( m.get(m.right(5)) ); // unreachable
}
}Two methods checkHeap(i) and exchange(i,j) are added. checkHeap(i) checks if the element at indices i, left(i) and right(i) to determine which the index of the the smallest element.
- If index i itself is returned, no exchanges are necessary.
- If left(i) is returned, then elements at i and left(i) need to be exchanged.
- If right(i) is returned, then elements at i and right(i) need to be exchanged.
exchange(i,j) swaps the elements at indices i and j if check(i) returned j such that j != i.
// MinHeap.java
public class MinHeap {
int[] arr;
public MinHeap(int[] keys) { ... }
public void print() { ... }
public String toString() { ... }
public int get(int i) { ... }
public int parent(int i) { ... }
public int left(int i) { ... }
public int right(int i) { ... }
/** check indices i, left(i) & right(i) and return the index with smallest element */
public int checkProperty(int i) {
if (left(i) == -1 && right(i) == -1)
return i;
else if (right(i) == -1) {
if (arr[i] < arr[left(i)])
return i;
else
return left(i);
}
else if (arr[i] < arr[left(i)] && arr[i] < arr[right(i)])
return i;
else if (arr[i] > arr[left(i)] && arr[i] < arr[right(i)])
return left(i);
else if (arr[i] < arr[left(i)] && arr[i] > arr[right(i)])
return right(i);
else if (arr[left(i)] < arr[right(i)])
return left(i);
else
return right(i);
}
/** Swap the elements at indices i and j */
void exchange(int i, int j) {
int temp = arr[i];
arr[i] = arr[j];
arr[j] = temp;
}
}// MinHeapTest.java
public class MinHeapTest {
public static void main(String[] args) {
int[] keys = {5, 3, 8, 6, 2, 1, 7, 9, 4, 0};
MinHeap m = new MinHeap(keys);
m.print(); // 5 3 8 6 2 1 7 9 4 0
int j = m.checkProperty(0);
if (j != 0)
m.exchange(0,j);
m.print(); // prints 3 5 8 6 2 1 7 9 4 0
System.out.println( m.checkProperty(2) ); // prints 5
}
}Now only thing left to build heap is to start fixing the nodes which do not adhere to heap property. Two points to remember at this stage.
- Leaf nodes need no fixing since they don't have children. Given that there are n nodes, only nodes the 0 to (n-2)/2 needs to be fixed. Rest will be leaf nodes. The last node is at index n-1. This implies its parent can be found at index (n-1-1)/2 = (n-2)/2.
- Any exchange at higher level nodes can cause the subtree under them to become lose the heap property, one exchange is not sufficient. To this end, we define a fixHeap method to ensure the entire subtree under a node is fixed recursively.
// MinHeap.java
public class MinHeap {
int[] arr;
public MinHeap(int[] keys) { ... }
public void print() { ... }
public String toString() { ... }
public int get(int i) { ... }
public int parent(int i) { ... }
public int left(int i) { ... }
public int right(int i) { ... }
public int checkProperty(int i) { ... }
void exchange(int i, int j) { ... }
/** Ensures the entire subtree under index i is fixed */
void fixHeap(int i) {
int j = checkProperty(i);
if (i == j) // arr[i] < arr[left(i)], arr[right(i)]
return; // Nothing to do. Subtree under i is heap.
else {
exchange(i,j); // can make subtree under j unstable
fixHeap(j); // fix the underneath node j recursively
}
}
/** Start fixing from last parent till root */
void buildHeap() {
for (int i=(arr.length-2)/2; i>=0; i--)
fixHeap(i);
}// MinHeapTest.java
public class MinHeapTest {
public static void main(String[] args) {
int[] keys = {5, 3, 8, 6, 2, 1, 7, 9, 4, 0};
MinHeap m = new MinHeap(keys);
m.print(); // prints 5 3 8 6 2 1 7 9 4 0
m.buildHeap(); // build the heap and print it
m.print(); // prints 0 2 1 4 3 8 7 9 6 5
}
}The minimum element is sitting at the root. i.e. index 0. Once the element at index 0 (root) is removed, the last element of the heap is put in the root's position. Since, this will cause the heap property to be lost at the root level, fixHeap(0) can be called to ensure the tree is turned into a heap once again. In other words, fixHeap(0) triggers some exchanges leading to the next minimum element reaching the root and the heap property satisfied by all nodes.
// MinHeap.java
public class MinHeap {
int[] arr;
public MinHeap(int[] keys) { ... }
public void print() { ... }
public String toString() { ... }
public int get(int i) { ... }
public int parent(int i) { ... }
public int left(int i) { ... }
public int right(int i) { ... }
public int checkProperty(int i) { ... }
void exchange(int i, int j) { ... }
void fixHeap(int i) { ... }
void buildHeap() { ... }
int extractMin() {
int val = arr[0]; // First copy the value at root
arr[0] = arr[arr.length-1]; // Bring last element to root
fixHeap(0); // Fix the heap from the root
return val;
} // MinHeapTest.java
public class MinHeapTest {
public static void main(String[] args) {
int[] keys = {5, 3, 8, 6, 2, 1, 7, 9, 4, 0};
MinHeap m = new MinHeap(keys);
m.print(); // prints 5 3 8 6 2 1 7 9 4 0
m.buildHeap(); // build the heap and print it
m.print(); // prints 0 2 1 4 3 8 7 9 6 5
System.out.println( m.extractMin() );
m.print(); // prints 1 2 5 4 3 8 7 9 6 5
// The 5 at the end is duplicate and invalid
}
}Note: The extractMin causes the heap size to reduce by one. This implies, the last element of the arr is invalid. The print method, in its current implementation, will print it. Since there is no way to shrink arr, a way to handle this scenario is to define an attribute size to track the current heap size. As extractMin is called each time, the size is decremented. The print function can be modified to print elements upto size and not till arr.length. In fact, all methods that use arr.length must be modified to use size instead of arr.length. i.e. print, toString, left, right, buildHeap and extractMin. The size can be initialized to arr.length in the constructor.
The complete program with references to arr.length replaced by size is given below.
// MinHeap.java
public class MinHeap {
int[] arr;
int size; // Tracks the size of the heap
/** Initializes the array object */
public MinHeap(int[] keys) {
arr = keys; // Make arr point to keys array
size = arr.length; // initialize size to arr.length
}
/** An utility method to print contents of MinHeap object */
public void print() {
System.out.print(arr[0]);
for (int i=1; i<size; i++)
System.out.print(" " + arr[i]);
System.out.println();
}
/** This function is used to stringfy the object */
public String toString() {
StringBuffer sb = new StringBuffer();
sb.append(arr[0]);
for (int i=1; i< size; i++)
sb.append(" " + arr[i]);
//sb.append("\n");
return sb.toString();
}
/** Retrieve the value at index i */
public int get(int i) {
return arr[i];
}
/** Given index i, retrieve the index of its parent */
public int parent(int i) {
// Two cases to handle - root and non-root cases
if (i == 0) // root node
return 0;
else
return (i-1)/2;
/* Note: if i==0, (i-1)/2 = -1/2 = 0.
So last statement alone is enough */
}
/** Given index i, retrieve the index of its left child */
public int left(int i) {
// Two cases to handle - internal and leaf nodes
if ( 2*i+1 < size ) // 1. internal node
return 2*i+1;
else // 2. leaf node
return -1;
}
/** Given index i, retrieve the index of its right child */
public int right(int i) {
// Two cases to handle - internal and leaf nodes
if ( 2*i+2 < size ) // 1. internal node
return 2*i+2;
else // 2. leaf node
return -1;
}
/** check indices i, left(i) & right(i) and return the index with smallest element */
public int checkProperty(int i) {
if (left(i) == -1 && right(i) == -1)
return i;
else if (right(i) == -1) {
if (arr[i] < arr[left(i)])
return i;
else
return left(i);
}
else if (arr[i] < arr[left(i)] && arr[i] < arr[right(i)])
return i;
else if (arr[i] > arr[left(i)] && arr[i] < arr[right(i)])
return left(i);
else if (arr[i] < arr[left(i)] && arr[i] > arr[right(i)])
return right(i);
else if (arr[left(i)] < arr[right(i)])
return left(i);
else
return right(i);
}
/** Swap the elements at indices i and j */
void exchange(int i, int j) {
int temp = arr[i];
arr[i] = arr[j];
arr[j] = temp;
}
/** Ensures the entire subtree under index i is fixed */
void fixHeap(int i) {
int j = checkProperty(i);
if (i == j)
return;
else {
exchange(i,j);
fixHeap(j);
}
}
/** Start fixing from last parent till root */
void buildHeap() {
for (int i=(size-2)/2; i>=0; i--)
fixHeap(i);
}
/** Retrieve the minimum element - that is at root (index 0) */
int extractMin() {
int val = arr[0]; // First copy the value at root
arr[0] = arr[size-1]; // Bring last element to root
fixHeap(0); // Fix the heap from the root
size--; // Decrement the size
return val;
}
}// MinHeapTest.java
public class MinHeapTest {
public static void main(String[] args) {
int[] keys = {5, 3, 8, 6, 2, 1, 7, 9, 4, 0};
MinHeap m = new MinHeap(keys);
m.print(); // prints the above array
m.buildHeap();
m.print(); // prints 0 2 1 4 3 8 7 9 6 5
System.out.println( m.extractMin() ); // prints 0
m.print(); // prints 1 2 5 4 3 8 7 9 6
// only 9 elements are printed now
}
}A way to test the program with different inputs is to use Random class to decide both the size and contents of the array. The test driver can be modified to this effect, so that every run of the program will greate a different series. Also, in order to check if the buildHeap really did construct the heap right, we can use the checkProperty on every internal node which should return the same index that it took as a parameter confirming parent < left, right always.
// MinHeapRandomTest.java
import java.util.Random;
public class MinHeapRandomTest {
public static void main(String[] args) {
//int[] keys = {5, 3, 8, 6, 2, 1, 7, 9, 4, 0};
// 1. Decide the size randomly
Random r = new Random();
int size = r.nextInt(100)+1; // Any number between 1-100
int[] keys = new int[size]; // create an array of such size
// 2. Populate the array with random elements
for (int i=0; i<size; i++)
keys[i] = r.nextInt(100); // Any number between 0-99
// 3. Build the heap
MinHeap m = new MinHeap(keys);
m.print(); // prints the above array
m.buildHeap();
m.print(); // prints the heap constructed
// 4. Check if every internal node satisfied heap property
for (int i=0; i<=(size-2)/2; i++) {
if ( m.right(i) != -1 ) { // if both left and right child exists
if ( m.get(i) > m.get(m.left(i)) || m.get(i) > m.get(m.right(i)) ) {
System.out.println("Test failed");
return;
}
}
else { // if left child alone exists
if ( m.get(i) > m.get(m.left(i)) ) {
System.out.println("Test failed");
return;
}
}
}
System.out.println("Test passed");
}
}C:\Users\swaminathanj\dsa\code>java MinHeapRandomTest
66 8 71 36 67 81 3 4 39 50 45 85 76 39 67 38 15 59 94 89 93 31 51 38 62 89 2 56 16 20 81 4 54 10 90 18 50 98 30 71 49
2 4 3 4 31 38 16 8 18 49 45 62 76 39 20 36 10 39 30 50 93 67 51 85 71 89 81 56 66 67 81 38 54 15 90 59 50 98 94 71 89
Test passed https://github.com/coding-guru-org/DSA_in_JAVA
26 12 94 43 95 81 79 94 8 53 8 7 36 80 7 39 12 92 76 46 29 92 2 61 86 6 70 90
2 8 6 8 12 7 7 12 43 29 26 61 36 80 79 39 94 92 76 46 53 92 95 94 86 81 70 90
Test passed
https://github.com/coding-guru-org/DSA_in_JAVA
26 63 31 65 37 29 51 85 20 60 36 18 49 34 14 15 53 3 53 55 47 13 66 20 8 30 45 50 80 20 60 42 75 56 95 47 91 51 18 6 62 33 50 62 3 0 95 34 96 29 96 0 11 95 92 28 16 5
0 0 5 3 3 8 14 15 18 6 13 18 11 16 20 42 53 20 51 55 33 37 36 20 29 29 45 28 34 26 60 85 75 56 95 47 91 65 53 60 62 47 50 62 63 66 95 34 96 31 96 30 49 95 92 51 50 80
Test passed
https://github.com/coding-guru-org/DSA_in_JAVA
8 58 52 25 55 83 73 16 58 6 91 79 37 63 95 31 68 65 50 98 92 21 4 40 56 39
4 6 37 16 8 39 63 25 50 58 21 40 52 73 95 31 68 65 58 98 92 55 91 79 56 83
Test passed
https://github.com/coding-guru-org/DSA_in_JAVA
42 92 9 57 53 67 49 35 56 49 85 81
9 35 42 56 49 67 49 57 92 53 85 81
Test passed
https://github.com/coding-guru-org/DSA_in_JAVA
0 67 3 37 92 35 10 50 75 79 38 12 16 17 46 15 19 88 19 80 29 7 64 42 99 61 86 71 64 52 56 53 79 66 62 87 30 46 2 94 26 56 8 33 91 52 61 8 6 8 83 2 46 49 4 54 61 87 72 28 9 15 52 2 8 66 89 57
0 2 2 2 7 3 9 8 19 8 33 6 4 17 10 15 19 30 37 26 29 38 52 8 8 16 35 54 64 28 15 50 66 57 62 87 88 46 75 94 80 56 79 92 91 64 61 12 42 99 83 61 46 49 86 71 61 87 72 46 52 56 52 53 67 79 89 66
Test passed https://github.com/coding-guru-org/DSA_in_JAVA
55 3 94 66 79 59 28 15 9 69 18 62 63 4 58 74 56 84 81 26 19 43 89 3 18 51 77 94 37 90 38 58 45 96 25 51 50 54 60 14 97 84 30 85 15 72 91 19 51 19 99 25 89 1 18 44 9 53 6 67 79 27 70 14 74 17 53 77
1 3 3 9 14 18 4 14 50 19 15 18 25 6 27 15 25 51 54 26 30 18 72 19 19 51 59 9 28 67 38 58 17 77 56 66 84 81 60 69 97 84 79 85 43 89 91 62 51 94 99 55 89 77 63 44 94 53 37 90 79 58 70 74 74 45 53 96
Test passed
C:\Users\swaminathanj\dsa\code>java MinHeapRandomTest
73 28 48 8 81 29 15 21 1 84 79 26 59 76 68 0 63 4 73 7 30 76 61 20 7 49 85 7 78 31 39 43 76 55 82 36 32 58 98 23 9 81 58 76 72 17 79 66 90 74 63 53 51 12 46 41 98 86 98 80 53 26 50
0 1 7 4 7 7 15 8 28 9 17 20 12 41 26 21 55 32 58 23 30 72 61 29 26 49 46 48 78 31 39 43 76 63 82 36 73 73 98 81 84 81 58 76 76 79 79 66 90 74 63 53 51 85 59 76 98 86 98 80 53 68 50
Test passed