【基础算法】- 堆排序

public class HeapSort {

	public static void main(String[] args) {
		
		HeapSort sort = new HeapSort();
		int[] a = {2,4,3,1,0,9,5,6,3,7};
		sort.sort(a);
		for(int i = 0 ; i < a.length ; i++){
			System.out.println(a[i]);
		}
	}
	
	private void maxHeapify(int[] a,int i,int length){
		
		int left = left(i);
		int right = right(i);
		int max = a[i];
		int index = i;
		if(left < length && max < a[left]){
			max = a[left];
			index = left;
		}
		if(right < length && max < a[right]){
			max = a[right];
			index = right;
		}
		if(index != i){
			a[i]^=a[index]; a[index]^=a[i]; a[i]^=a[index];
			maxHeapify(a,index,length);
		}
	}
	
	private void buildMaxHeap(int[] a){
		
		for(int i = a.length / 2 - 1 ; i >= 0 ; i--){
			maxHeapify(a,i,a.length);
		}
	}
	
	private void sort(int[] a){
		
		buildMaxHeap(a);
		for(int i = a.length - 1 ; i >= 0 ; i--){
			if(i != 0){
				a[i]^=a[0]; a[0]^=a[i]; a[i]^=a[0];
			}
			maxHeapify(a,0,i);
		}
	}
	
	private int left(int i){
		return i*2;
	}
	
	private int right(int j){
		return j*2+1;
	}
}