面试题15 查找最小的 k 个元素 [数组] / (堆优化 STL O(nlogk) )[STL]

平均复杂度 O(nlogk) 的基于快排的方法

#include <iostream>
#include <cstdio>
#include <cstring>
#include <stack>
#include <algorithm>
#define BUG cout << "here\n";
using namespace std;
const int N = 105;

int a[N] = {4, 5, 1, 6, 2, 7, 3, 8};
int partition(int a[], int left, int right) {
    if(left < right) {
        int l = left, r = right, x = a[l];
        while(1) {
            while(l < r && a[r] >= x) r--;
            while(l < r && a[l] <= x) l++;
            if(l >= r) break;
            swap(a[r], a[l]);
        }
        swap(a[left], a[l]);
        return l;
    }
    else return left;
}
void solve(int* input, int n, int k) {
    if(input == NULL || k > n || n <= 0 || k <= 0) {
        return;
    }
    int start = 0;
    int end = n - 1;
    int index = partition(input, start, end);
    while(index != k-1) {
        if(index > k-1) {
            end = index - 1;
            index = partition(input, start, end);
        }
        else {
            start = index + 1;
            index = partition(input, start, end);
        }
    }
    for(int i = 0; i < k; i++) {
        cout << input[i] << ' ';
    }
    cout << endl;
}
int main() {
    solve(a, 8, 5);
    return 0;
}

(堆优化 STL O(nlogk) ) 【STL】 

#include <iostream>
#include <string>
#include <cstdio>
#include <set>
#include <vector>
#include <functional>
#include <algorithm>
using namespace std;

typedef multiset<int, greater<int> > intSet; /// 定义容器的
typedef multiset<int, greater<int> >::iterator setIterator;  /// 定义迭代器的

void getLastNumbers(const vector<int>& data, intSet& setK, int k) {
    setK.clear();
    if(k < 1 || data.size() < k) return;
    vector<int>::const_iterator iter = data.begin();
    for(;iter != data.end(); ++iter) {
        if(setK.size() < k) {
            setK.insert(*iter);
        }
        else {
            setIterator iter2 = setK.begin(); // 最大的位置
            if(*iter < (*iter2)) {
                setK.erase(iter2);
                setK.insert(*iter);
            }
        }
    }
    setIterator sit = setK.begin();
    for(;sit != setK.end(); sit++) {
        cout << *sit << ' ';
    }
    cout << endl;
}
int main() {
    vector<int> V;
    V.push_back(4); V.push_back(5); V.push_back(1); V.push_back(6);
    V.push_back(2); V.push_back(7); V.push_back(3); V.push_back(8);
    intSet ms;
    getLastNumbers(V, ms, 5);
    return 0;
}