heap sort(hdu 1425)

http://acm.hdu.edu.cn/showproblem.php?pid=1425

题意：给你n个整数，请按从大到小的顺序输出其中前m大的数。

一看题目，就觉得是用堆排序，如果在n大，m比较小的时候，效率会不错，空间方面：O(m)， 时间基本是O(n),当前前题是 m比较小，

如果很大的话，就回到了O(nlgn),

代码如下：

heap sort: time:718MS

#include<iostream>
using namespace std;
const int N = 1000005;
int a[N];
//index from 0,1,2,3,4.....
int Left(int curr){
    return (curr<<1)+1;
}
int Right(int curr){
    return (curr<<1)+2;
}
//update the tree with root: p[node], make it a min heap
void minHeapify(int p[], int node, int size){
    //printf("curr:%d, size:%d\n", node, size);
    int left, right, tmp;
    int curr = node;
    //update
    while( Left(curr) < size-1 ){
        left = Left(curr);
        right = Right(curr);
        //printf("curr:%d,left:%d, right:%d\n",curr, left, right);
        if(p[left] < p[right] ){
            if( p[curr] > p[left] ){
                tmp = p[curr];
                p[curr] = p[left];
                p[left] = tmp;
                curr = left;
            }
            else break;
        }
        else{
            if( p[curr] > p[right] ){
                tmp = p[curr];
                p[curr] = p[right];
                p[right] = tmp;
                curr = right;
            }
            else break;
        }
    }//end of while
    //if has left node as leaf,then update
    if(Left(curr) == size-1 && p[curr] > p[Left(curr)]){
            tmp = p[curr];
            p[curr] = p[Left(curr)];
            p[Left(curr)] = tmp;
    }
}//end of minHeapify

void buildMinHeap(int p[], int size){
    for(int i= (size-2)/2; i>=0; i--){
        minHeapify(p, i, size);
    }
}
int main()
{
    int n, m, i, u, k;
    int tmp;
    while(cin >> n >> m ){
        for(i=0; i<m; i++){
            scanf("%d", &a[i]);
        }
        //build heap
        buildMinHeap(a, m);
        
        //printf("before:\n");
        //for(i=0; i<m; i++) printf("%d ", a[i]);    
    
        //process other data
        for(i=m; i<n; i++){
            scanf("%d", &u);
            if(u > a[0]){
                a[0] = u;
                minHeapify(a,0, m);
            }
        }
        //printf("after:\n");
        //for(i=0; i<m; i++) printf("%d ", a[i]);    
        //printf("------------:\n");
        
        //print the m max data
        k = m-1;
        for(i=0; i<m-1; i++){
            tmp = a[k];
            a[k] = a[0];
            a[0] = tmp;
            minHeapify(a, 0, k);
            k--; 
        }
        
        printf("%d", a[0]);
        for(i=1; i<m; ++i){
            printf(" %d", a[i]);
        }
        printf("\n");
    }
}

标准库的快排 sort(): time:937MS

#include<iostream>
#include<algorithm>
using namespace std;
const int N = 1000005;
int a[N];


bool cmp(int a, int b){
    return a>b;
}

int main()
{
    int n, m, i, u, k;
    int tmp;
    while(cin >> n >> m ){
        for(i=0; i<n; i++){
            scanf("%d", &a[i]);
        }
    
        //use stdlib sort
        sort(a, a+n, cmp);            
        //output the result
        printf("%d", a[0]);
        for(i=1; i<m; ++i){
            printf(" %d", a[i]);
        }
        printf("\n");
    }
}

标准库中的 partial_sort(): time:734MS

#include<iostream>
#include<algorithm>
using namespace std;
const int N = 1000005;
int a[N];

bool cmp(int a, int b){
    return a>b;
}

int main()
{
    int n, m, i, u, k;
    int tmp;
    while(cin >> n >> m ){
        for(i=0; i<n; i++){
            scanf("%d", &a[i]);
        }
    
        //use stdlib sort
        //sort(a, a+n, cmp);            
        partial_sort(a, a+m, a+n, cmp);
        //output the result
        printf("%d", a[0]);
        for(i=1; i<m; ++i){
            printf(" %d", a[i]);
        }
        printf("\n");
    }
}