LintCode-Heapify

LiBlog發表於2014-12-29
API

Given an integer array, heapify it into a min-heap array.

For a heap array A, A[0] is the root of heap, and for each A[i], A[i * 2 + 1] is the left child of A[i] and A[i * 2 + 2] is the right child of A[i].
Example

Given [3,2,1,4,5], return [1,2,3,4,5] or any legal heap array.

Challenge

O(n) time complexity

Clarification

What is heap?

  • Heap is a data structure, which usually have three methods: push, pop and top. where "push" add a new element the heap, "pop" delete the minimum/maximum element in the heap, "top" return the minimum/maximum element.
 
What is heapify?
  • Convert an unordered integer array into a heap array. If it is min-heap, for each element A[i], we will get A[i * 2 + 1] >= A[i] and A[i * 2 + 2] >= A[i].
 
What if there is a lot of solutions?
  • Return any of them.

Solution:

I implemented a Heap class that can specify min heap or max heap with insert, delete root and build heap functions.

  1 class Heap{
  2     private int[] nodes;
  3     private int size;
  4     private boolean isMaxHeap;
  5     
  6     public Heap(int capa, boolean isMax){
  7         nodes = new int[capa];
  8         size = 0;
  9         isMaxHeap = isMax;
 10     }
 11 
 12     //Build heap from given array.
 13     public Heap(int[] A, boolean isMax){
 14         nodes = new int[A.length];
 15         size = A.length;
 16         isMaxHeap = isMax;
 17         for (int i=0;i<A.length;i++) nodes[i] = A[i];
 18         int start = A.length/2;
 19         for (int i=start;i>=0;i--)
 20             shiftDown(i);
 21     }
 22 
 23     //Assume A and nodes have the same length.
 24     public void getNodesValue(int[] A){
 25         for (int i=0;i<nodes.length;i++) A[i] = nodes[i];
 26     }    
 27     
 28     public boolean isEmpty(){
 29         if (size==0) return true;
 30         else return false;
 31     }
 32 
 33     public int getHeapRootValue(){
 34         //should throw exception when size==0;
 35         return nodes[0];
 36     }
 37 
 38     private void swap(int x, int y){
 39         int temp = nodes[x];
 40         nodes[x] = nodes[y];
 41         nodes[y] = temp;
 42     }
 43 
 44     public boolean insert(int val){
 45         if (size==nodes.length) return false;
 46         size++;
 47         nodes[size-1]=val;
 48         //check its father iteratively.
 49         int cur = size-1;
 50         int father = (cur-1)/2;
 51         while (father>=0 && ((isMaxHeap && nodes[cur]>nodes[father]) || (!isMaxHeap && nodes[cur]<nodes[father]))){
 52             swap(cur,father);
 53             cur = father;
 54             father = (cur-1)/2;
 55         }
 56         return true;
 57     }
 58 
 59     private void shiftDown(int ind){
 60         int left = (ind+1)*2-1;
 61         int right = (ind+1)*2;
 62         while (left<size || right<size){
 63             if (isMaxHeap){
 64                 int leftVal = (left<size) ? nodes[left] : Integer.MIN_VALUE;
 65                 int rightVal = (right<size) ? nodes[right] : Integer.MIN_VALUE;
 66                 int next = (leftVal>=rightVal) ? left : right;
 67                 if (nodes[ind]>nodes[next]) break;
 68                 else {
 69                     swap(ind,next);
 70                     ind = next;
 71                     left = (ind+1)*2-1;
 72                     right = (ind+1)*2;
 73                 }
 74             } else {
 75                 int leftVal = (left<size) ? nodes[left] : Integer.MAX_VALUE;
 76                 int rightVal = (right<size) ? nodes[right] : Integer.MAX_VALUE;
 77                 int next = (leftVal<=rightVal) ? left : right;
 78                 if (nodes[ind]<nodes[next]) break;
 79                 else {
 80                     swap(ind,next);
 81                     ind = next;
 82                     left = (ind+1)*2-1;
 83                     right = (ind+1)*2;
 84                 }
 85             }
 86         }
 87     }    
 88 
 89     public int popHeapRoot(){
 90         //should throw exception, when heap is empty.
 91 
 92         int rootVal = nodes[0];
 93         swap(0,size-1);
 94         size--;
 95         if (size>0) shiftDown(0);
 96         return rootVal;
 97     }
 98 }
 99         
100             
101     
102 
103 public class Solution {
104     /**
105      * @param A: Given an integer array
106      * @return: void
107      */
108     public void heapify(int[] A) {
109         if (A.length==0) return;
110     
111         Heap minHeap = new Heap(A,false);
112         minHeap.getNodesValue(A);
113     }
114 }