为算法考试做准备--快速排序以及找第K大数的实现

最新推荐文章于 2025-03-30 18:26:14 发布

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最新推荐文章于 2025-03-30 18:26:14 发布

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分类专栏：数据结构与算法分析Java版文章标签：快速排序算法

本文链接：https://blog.youkuaiyun.com/zhangyingli/article/details/48086247

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本文详细介绍了快速排序算法的实现过程，包括插入操作、快速排序递归、分区操作及查找第K小元素的方法，并通过实例展示了算法的应用。

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package utils;

import java.util.Random;

class ArrayInts{
	private int[] theArray;
	private int nElems;
	
	public ArrayInts(int maxSize) {
		theArray = new int[maxSize];
		nElems = 0;
	}
	
	public void insert(int value) {
		theArray[nElems] = value;
		nElems++;
	}
	
	public void quickSort() {
		recQuickSort(0, nElems - 1);
	}
	
	public void recQuickSort(int left, int right) {
		if (right - left <= 0) {
			return;
		} 
		
		int pivot = theArray[right];		
		int partition = partitionIt(left, right, pivot);
		recQuickSort(left, partition - 1);
		recQuickSort(partition + 1, right);
	}
	
	public int partitionIt(int left, int right, int pivot) {
		int leftPtr = left - 1;
		int rightPtr = right;
		while (true) {
			while (theArray[++leftPtr] < pivot)
				;
			while (rightPtr > 0 && theArray[--rightPtr] > pivot)
				;
			if (leftPtr >= rightPtr)
				break;
			else
				swap(leftPtr, rightPtr);
		}
		swap(leftPtr, right);
		return leftPtr;
	}
	
	public void swap(int i1, int i2) {
		int temp = theArray[i1];
		theArray[i1] = theArray[i2];
		theArray[i2] = temp;
	}
	
	public int findKth(int left, int right, int k) {
		if (right == left)
			return theArray[left];
		int pivot = theArray[right];
		int partition = partitionIt(left, right, pivot);
		int i = partition - left;
		if (i == k)
			return theArray[k];
		else if (i < k)
			return findKth(partition + 1, right, k - i);
		else
			return findKth(left, partition - 1, k);
	}
	
	public void display() {
		for (int j = 0; j < nElems; ++j)
			System.out.print(theArray[j] + " ");
		System.out.println();
	}
}
public class MyQuickSort {
	public static void main(String[] args) {
		ArrayInts ints = new ArrayInts(20);
		Random rand = new Random(47);
		for (int i = 0; i < 20; ++i) {
			ints.insert(rand.nextInt(100) + 1);
		}
		System.out.println("find 0th: " + ints.findKth(0, 19, 0));
		System.out.println("find 19th: " + ints.findKth(0, 19, 19));
		System.out.println("find 9th: " + ints.findKth(0, 19, 9));
		System.out.println("find 13th: " + ints.findKth(0, 19, 14));
		ints.quickSort();
		ints.display();
	}
}