612.1.002 ALGS4 | Analysis of Algorithms

我们生活在大数的时代

培养数量级的敏感！

Tip：见招拆招

• 作为工程师，你先要能实现出来。
• 充实基础，没有什么不好意思
• 哪怕不完美。但是有时候完成比完美更重要。
• 之后再去想优化

P.S.作者Robert Sedgewick的导师是Knuth（高德纳！）

Conclusion First

1.Running Time

• Operation table

2.Memory

1 SOP - Analysis

2 Observations

• Measuring the running time - automatic

public class Stopwatch(part of stdlib.jar/algs4.course)

    public static void main(String[] args) {
        In in = new In(args[0]);
        int[] a = in.readAllInts();

        Stopwatch timer = new Stopwatch();
        int count = count(a);
        StdOut.println("elapsed time = " + timer.elapsedTime());//time since creation (in seconds)
        StdOut.println(count);
    }

3 Mathematical Model - Knuth(高德纳！)

Simplification 1: cost model

Simplification 2: tilde notation

approximate

工程近似

• Bottom line. Use cost model and tilde notation to simplify counts.

4 Order of growth

• Operation table

5 Binary Search - code 二分搜索

不看代码自己写

public static int binarySearch(int[]a,int key)
        {
        int lo=0;
        int hi=a.length-1;
        while(lo<=hi){
            int mid=lo+(hi-lo)/2;
            if(key>a[mid])lo=mid+1;
            else if(key<a[mid])hi=mid-1;
            else return mid;
        }
        return-1;
        }

6 Memory

Typical memory usage of Java

Object overhead - 对象开销

QuickUnion