Data Structures & Algorithms in Java ---- Arrays

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最新推荐文章于 2024-08-27 09:08:07 发布

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分类专栏： Java Algorithm

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本文深入探讨了Java中数组的特性及其在不同操作中的效率差异，包括数组的创建、插入、搜索、删除以及如何通过封装保护数组免受无意修改。同时，文章详细介绍了算法效率的度量方式——Big O 表示法，以及线性搜索、二分搜索和它们的时间复杂度对比，最终以Big O符号提供了一种比较算法速度的有效方法。

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1、Summary

• Arrays in Java are objects, created with the new operator.

• Unordered arrays offer fast insertion but slow searching and deletion.

• Wrapping an array in a class protects the array from being inadvertently altered.

• A class interface comprises the methods (and occasionally fields) that the class user can access.

• A class interface can be designed to make things simple for the class user.

• A binary search can be applied to an ordered array.

• The logarithm to the base B of a number A is (roughly) the number of times you can divide A by B before the result is less than 1.

• Linear searches require time proportional to the number of items in an array.

• Binary searches require time proportional to the logarithm of the number of items.

• Big O notation provides a convenient way to compare the speed of algorithms.

• An algorithm that runs in O(1) time is the best, O(log N) is good, O(N) is fair, and O(N2)  is pretty bad.

2、Big O notation

Automobiles are divided by size into several categories: subcompacts, compacts, midsize, and so on. These categories provide a quick idea what size car you're talking about, without needing to mention actual dimensions. Similarly, it's useful to have a shorthand way to say how efficient a computer algorithm is. In computer science, this rough measure is called Big O notation.

3、Binary Search with the find() Method

	public int find(double searchKey)
	{
		int lowerBound = 0;

		int upperBound = nElems - 1;

		int curIn;

		while (true)
		{
			curIn = (lowerBound + upperBound) / 2;

			if (a[curIn] == searchKey)
			{
				return curIn; // found it
			}
			else if (lowerBound > upperBound)
			{
				return nElems; // can't find it
			}
			else // divide range
			{
				if (a[curIn] < searchKey)
				{
					lowerBound = curIn + 1; // it's in upper half
				}
				else
				{
					upperBound = curIn - 1; // it's in lower half
				}

			} // end else divide range

		} // end while

	} // end find()