算法——快速排序

public class Quick {

    public static void sort(Comparable[] a){
        sort(a,0,a.length-1);
    }

    public static void sort(Comparable[] a,int lo,int hi){
        if(hi <= lo ) return;
        int j = partition(a,lo,hi);
        sort(a,lo,j - 1);
        sort(a,j + 1,hi);
    }

    public static int partition(Comparable[] a,int lo,int hi){
        int i = lo;
        int j = hi + 1;
        Comparable v = a[lo];

        while(true){
            while(less(a[++i],v)) if( i== hi) break;
            while(less(v,a[--j])) if( j== lo ) break;
            if(i >= j) break;
            echo(a,i,j);
        }
        echo(a,lo,j);
        return j;
    }


    public static boolean less(Comparable v,Comparable w){
        return v.compareTo(w) < 0;
    }

    public static void echo(Comparable[] a,int i,int j){
        Comparable t = a[i];
        a[i] = a[j];
        a[j] = t;
    }

    public static void show(Comparable[] a){
        for(int i = 0; i < a.length;i++ ){
            System.out.print(a[i] + " ");
        }
    }

    public static boolean isSorted(Comparable[] a){
        for(int i = 1; i < a.length;i++){
            if(less(a[i],a[i-1])) return false;
        }
        return true;
    }

    public static Comparable[] Random(int num){
        Integer[] a = new Integer[num];
        Random rand = new Random();
        for(int i = 0;i<num;i++){
            a[i] =  rand.nextInt(100000);
        }
        return a;
    }

    public static void main(String[] args) {
        Comparable[] a = Selection.Random(20000000);
        /*show(a);*/
        Long start = System.currentTimeMillis();
        sort(a);
        Long end = System.currentTimeMillis();
        System.out.println("Time is " + (end - start));
        /*show(a);*/
    }
}

特点：每一次切分，就会找到一个正确值a[j]。
难点：在切分partition(),一般随意找a[lo]作为切分元素，比它小的在左边，比它大的在右边。 通过从左开始扫描，找到比它大的，从右扫描，找到比它小的，然后两者进行交换，直到i >= j才结束;在把a[lo]和a[j]交换。