TOP K 問題的多種實作方法

// Date   : 2016.10.25
// Author : yqtao
// https://github.com/yqtaowhu
#include <iostream>
#include<vector>
#include<set>
#include<algorithm>
#include<functional>
#include<queue>
using namespace std;
int partition(vector<int>& data, int left, int right) {
    int i = left, j = right, pivot = data[left];
    while (i < j) {
        while (i < j&&data[j] >= pivot) j--;
        data[i] = data[j];
        while (i < j&&data[i] <= pivot) i++;
        data[j] = data[i];
    }
    data[i] = pivot;
    return i;
}
vector<int> getLeastNums(vector<int>& data, int k) {
    int n = data.size();
    if (n <=  || k <=  || k > n) return vector<int>();
    int left = , right = n - ;
    int index = partition(data, left,right);
    while (index != k - ) {
        if (index > k - ) {
            right = index - ;
            index = partition(data, left, right);
        }
        else {
            left = index + ;
            index = partition(data, left, right);
        }
    }
    vector<int>res(data.begin(), data.begin() + k);
    return res;
}
//solution 2
//特别注意的一點multiset，和set的不同
//mulgiset可以具有重複值，而set不是，二者都是基于紅黑書實作的
vector<int> getLeastNums2(vector<int>& data, int k) {
    int n = data.size();
    if (n <=  || k <=  || k > n) return vector<int>();
    multiset<int,greater<int>> st;
    auto it = data.begin();
    for (; it != data.end(); ++it) {
        if (st.size() < k) st.insert(*it);
        else {
            auto iter = st.begin();
            if (*it < *iter) {
                st.erase(iter);
                st.insert(*it);
            }
        }
    }
    vector<int>res(st.begin(), st.end());
    return res; 
}
//優先隊列，本質是根實作的
vector<int> getLeastNums3(vector<int>& data, int k) {
    int n = data.size();
    if (n <=  || k <=  || k > n) return vector<int>();
    priority_queue<int,vector<int>,greater<int>> que;  //greater表示的是小根堆，一定要注意了
    for (int i = ; i < n; i++)
        que.push(data[i]);
    vector<int> res;
    while (res.size() < k) {
        res.push_back(que.top());
        que.pop();
    }
    return res;
}
int main() {
    vector<int> data = { ,,,,,,,,, };
    vector<int> res = getLeastNums3(data,);
    for (auto c : res)
        cout << c << " ";
    cout << endl;
}