输出前k大的数

1.快速排序

#include<iostream>#include<algorithm>using namespace std;int Median3(int A[],int Left,int Right){int Center=(Left+Right)/2;if(A[Left]>A[Center])  swap(A[Left],A[Center]);if(A[Left]>A[Right])  swap(A[Left],A[Right]);    if(A[Center]>A[Right])  swap(A[Center],A[Right]);swap(A[Center],A[Right-1]);return A[Right-1];}void InsertionSort(int A[],int N){int j,p;int temp;for(p=1;p<N;p++){temp=A[p];for(j=p;j>0&&A[j-1]>temp;j--)A[j]=A[j-1];A[j]=temp;}}#define cutoff 3void Qsort(int A[],int Left,int Right){int i,j;int pivot;if(Left+cutoff<=Right){pivot=Median3(A,Left,Right);i=Left;j=Right-1;for( ; ; ){while(A[++i]<pivot) {}while(A[--j]>pivot) {}if(i<j)swap(A[i],A[j]);elsebreak;}swap(A[i],A[Right-1]);Qsort(A,Left,i-1);Qsort(A,i+1,Right);}elseInsertionSort(A+Left,Right-Left+1);}void Quicksort(int A[],int N){Qsort(A,0,N-1);}int main(){int n,*A,k;cin>>n;A=new int[n];for(int i=0;i<n;i++)cin>>A[i];cin>>k;Quicksort(A,n);for(int i=n-1;i>=n-k;i--){cout<<A[i]<<endl;}return 0;}

2.快速排序的 partition 方法

#include<iostream>#include<cstdio>using namespace std;int k;int a[100005];void swap(int & a,int & b){    int tmp;    tmp = a;    a = b;    b = tmp;}void Qsort( int a[], int start, int end ){    if(start >= end){     return;    }    int pivot = a[start];    int i = start,j = end;    while (i < j)    {        while ( j > i && a[j] >= pivot )            --j;        swap( a[i], a[j]);        while ( i < j && a[i] <= pivot )            ++i;        swap( a[i], a[j]);    }    Qsort( a, start, i-1 );    Qsort( a, i+1, end );}void FindMaxK(int a[],int k, int start, int end){    if(start <= end)    {        int pivot = a[start];        int i = start, j = end;        while (i < j)        {            while ( j > i && a[j] >= pivot )                --j;            swap( a[i], a[j]);            while ( i < j && a[i] <= pivot )                ++i;            swap( a[i], a[j]);        }        if(end - i + 1 == k){        return;        }        else if(end - i + 1 > k)  {        FindMaxK(a,k,i+1,end);        }        else {        FindMaxK(a,k-(end-i+1),start,i-1);        }    }}int main(){    int size;    scanf("%d",&size);    for(int i=0;i < size;++i){    scanf("%d",&a[i]);    }    scanf("%d",&k);    FindMaxK(a,k,0,size-1);    Qsort(a,size-k,size-1);    for(int i = size-1;i >= size - k;--i){        printf("%d\n",a[i]);    }    return 0;}

3.最小堆

#include<iostream>#include<cstdio>using namespace std;int k;int a[100005];void swap(int & a,int & b){    int tmp;    tmp = a;    a = b;    b = tmp;}void PercDown( int A[], int p, int N ){    int Parent, Child;    int X;    X = A[p]; /* 取出根结点存放的值 */    for( Parent=p; (Parent*2+1)<N; Parent = Child ) {        Child = Parent * 2 + 1;        if( ( Child != N-1 ) && ( A[Child] > A[Child+1] ) ){            Child++;  /* Child指向左右子结点的较大者 */        }        if( X <= A[Child] ){        break; /* 找到了合适位置 */        }        else {  /* 下滤X */            A[Parent] = A[Child];        }    }    A[Parent] = X;}void InsertionSort(int A[],int N){int j,p;int temp;for(p=1;p<N;p++){temp=A[p];for(j=p;j>0&&A[j-1]>temp;j--)A[j]=A[j-1];A[j]=temp;}}int main(){    int size;    scanf("%d",&size);    for(int i=0;i < size;++i){    scanf("%d",&a[i]);    }    scanf("%d",&k);    int *arr ;    arr = new int[k];    for (int i = 0; i < k; ++i) {arr[i] = a[i];}//    建最小堆for (int var = size/2-1; var >= 0 ; --var) {PercDown(arr,var,k);}//检查剩余的size-k个数for (int i = k; i < size; ++i) {if(a[i] > arr[0]){arr[0] = a[i];PercDown(arr, 0 , k);}}InsertionSort(arr,k);    for(int i = k-1;i >= 0;--i){        printf("%d\n",arr[i]);    }    return 0;}