算法——归并排序

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public class Merge {    private static Comparable[] aue;    public static void sort(Comparable[] a){        aue = new Comparable[a.length];        sort(a,0,a.length-1);    }    public static void sort(Comparable[] a,int lo,int hi){        if(hi <= lo ) return;        int mid = (lo + hi)/2;        sort(a,lo,mid);        sort(a,mid+1,hi);        merge(a,lo,mid,hi);    }    public static void merge(Comparable[] a,int lo,int mid,int hi){        int i = lo;        int j = mid + 1;        for(int k = lo; k <= hi;k++){            aue[k] = a[k];        }        for(int k = lo; k <= hi;k++){            if( i > mid) a[k] = aue[j++];            else if( j > hi) a[k] = aue[i++];            else if(less(aue[j],aue[i])) a[k] = aue[j++];            else a[k] = aue[i++];        }    }    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(2000000);        /*show(a);*/        Long start = System.currentTimeMillis();        sort(a);        Long end = System.currentTimeMillis();        System.out.println("Time is " + (end - start));        /*show(a);*/    }}

思路:利用分治思想,把大问题拆分为小问题,然后通过小问题的答案来解决大问题的答案, 归并重点是分为两步,一个是拆分,通过递归不断的对半分,直到划分为一个数时就返回,另一个是合并,通过增加一个额外的数组,长度为N,利用循环进行4个判断,左半部分用尽就用右半部,反之亦然,右半边小于左半边的情况和右半边大于左半边的情况,这里要注意j=mid + 1,一定要加1,这样mid会往右边偏移,才不会出错。

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