1.4.15 ThreeSum

package analyse;
import java.util.Arrays;

import edu.princeton.cs.algs4.*;
import search.BinarySearch;
import tool.StopWatch;

public class ThreeSum {
    public static int count(int[] a){
        int count = 0;
        int len = a.length;
        
        for(int i = 0; i < len; i++){
            for(int j  = i + 1; j < len; j++){
                for(int z = j + 1; z < len; z++){
                    if((a[i] + a[j] + a[z]) == 0){
                        count++;
                    }
                }
            }
        }
        
        return count;
    }
    
    public static int countFaster(int[] a){
        int count = 0;
        int len = a.length;
        Arrays.sort(a); 
        
        for(int i = 0; i < len; i++){
            for(int j = i + 1; j < len; j++){
                if(j < BinarySearch.rank(-a[i]-a[j], a))
                    count++;
            }
        }   
        
        return count;
    }
    
    public static int countFastest(int[] a){
        int count = 0;
        Arrays.sort(a);
        int len = a.length;
        int posmin;
        int posmax;
    
        for(int i = 0; i < len; i++){
            posmin = i + 1;
            posmax = len - 1;
            while(posmin < posmax && a[posmin] + a[i] <= 0){
                if(a[posmin] + a[i] + a[posmax] < 0) posmin++;
                else if(a[posmin] + a[i] + a[posmax] > 0) posmax--;
                else {posmin++; posmax--; count++;}
            }
        }

        return count;
    }
    
    public static void main(String[] args) {
        //Attention: the array must has no duplicate elements;

        int a[] = new In("data/4Kints.txt").readAllInts();
        Stopwatch timer = new Stopwatch();
        System.out.print(count(a) + " ");
        System.out.println(timer.elapsedTime());
        
        Stopwatch timer2 = new Stopwatch();
        System.out.print(countFaster(a) + " ");
        System.out.println(timer2.elapsedTime());
        
        Stopwatch timer3 = new Stopwatch();
        System.out.print(countFastest(a) + " ");
        System.out.println(timer3.elapsedTime());
        
    }
}

来见证不同算法的性能差距吧吧：

Paste_Image.png