100w个数中找出最大的前k个数

//*******************100w个数中找出最大的前k个数***************************

//挨个遍历排序：100w个数太多，内存不够，而且遍历排序效率太低

//建大堆：只能选出最大的那个数

//可选方案：建K个数的小堆

//          首先取这100w个数中的前K个数建立最小堆，即长度为K的数组的首元素为最小的元素

//          然后依次取剩下的元素，和首元素进行比较，如果比首元素大则替换，然后继续排最

//          小堆，让最小的数排在首元素位置，继续比较

#defineM100000

#defineK100

//向下调整成最小堆

voidAdjustDown(intparent,int*a)

{

                intchild =parent* 2 + 1;

                while(child <K)

                {

                                if(child + 1 <K&&a[child] >a[child + 1])

                                {

                                                child++;

                                }

                                if(a[child] <a[parent])

                                {

                                                swap(a[child],a[parent]);

                                                parent= child;

                                                child =parent* 2 + 1;

                                }

                                else

                                {

                                                break;

                                }

                }

}

voidGetKMaxNum(intarray[],inttop[])

{

                assert(K<M);

                //取array数组里面的前K个数给top数组

                for(inti = 0; i <K; i++)

                {

                                top[i] =array[i];

                }

                //建最小堆

                for(inti = (K - 2) / 2; i >= 0;i--)

                {

                                AdjustDown(i,top);

                }

                //取array里面剩下的数和top里面的首元素进行比较，如果比首元素大，则替换，然后再次调整成最小堆

                for(inti = K; i <M; i++)

                {

                                if(array[i]>top[0])

                                {

                                                top[0] =array[i];

                                                AdjustDown(0,top);

                                }

                }

}

voidPrint(int*top)

{

                intcount = 0;

                for(inti = 0; i <K; i++)

                {

                                cout <<top[i] <<"   ";

                                count++;

                                if(count % 6==0 )

                                {

                                                cout << endl;

                                }

                }

}

voidTest()

{

                intarray[M] = { 0 };

                inttop[K] = { 0 };

                for(inti = 0; i <M; i++)

                {

                                array[i] = i;

                }

                array[9] = 1111111;

                array[99] = 11111111;

                array[999] = 111111111;

                GetKMaxNum(array, top);

                Print(top);

}

intmain()

{

                Test();

                return0;

}