算法之每日一题：最小的K个数

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#include <vector>#include <iostream>#include <algorithm>#include <string>#include <set>#include <functional>using namespace std;class Solution {public://使用简单粗暴的全部排序的方法并不能取得很好的性能，因为只要求k个数，所以使用最小堆就可以了。//(如果是要取最大值就是用最大堆）void heapSort(vector<int> &input, int root, int end){for (int j = end - 1; j >= root; j--){int parent = (j + root - 1) / 2;if (input[parent]>input[j]){int temp = input[j];input[j] = input[parent];input[parent] = temp;}}}//标准方法：堆排序vector<int> GetLeastNumbers_Solution_headSort_custom(vector<int> input, int k) {vector<int> result;if (k>input.size())return result;for (int i = 0; i<k; i++){heapSort(input, i, input.size());result.push_back(input[i]);}return result;}//方法二：全排序vector<int> GetLeastNumbers_Solution_allsort(vector<int> input, int k) {vector<int> res;if (input.empty() || k>input.size()) return res;sort(input.begin(), input.end());for (int i = 0; i<k; i++)res.push_back(input[i]);return res;}int Partition(vector<int>& input, int begin, int end){int low = begin;int high = end;int pivot = input[low];while (low<high){while (low<high&&pivot <= input[high])high--;input[low] = input[high];while (low<high&&pivot >= input[low])low++;input[high] = input[low];}input[low] = pivot;return low;}//方法三：Partiton思想vector<int> GetLeastNumbers_Solution_Partiton(vector<int> input, int k) {int len = input.size();if (len == 0 || k>len || k <= 0) return vector<int>();if (len == k) return input;int start = 0;int end = len - 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);}}vector<int> res(input.begin(), input.begin() + k);return res;}//堆排序vector<int> GetLeastNumbers_Solution_heapSort(vector<int> input, int k) {int len = input.size();if (len <= 0 || k>len || k <= 0) return vector<int>();vector<int> res(input.begin(), input.begin() + k);//建堆make_heap(res.begin(), res.end());for (int i = k; i<len; i++){if (input[i]<res[0]){//先pop,然后在容器中删除pop_heap(res.begin(), res.end());res.pop_back();//先在容器中加入，再pushres.push_back(input[i]);push_heap(res.begin(), res.end());}}//使其从小到大输出sort_heap(res.begin(), res.end());return res;}//红黑树：multiset集合  利用仿函数改变排序顺序 时间复杂度O（nlogk） vector<int> GetLeastNumbers_Solution(vector<int> input, int k) {int len = input.size();if (len <= 0 || k>len || k <= 0) return vector<int>();//仿函数中的greater<T>模板，从大到小排序multiset<int, greater<int> > leastNums;vector<int>::iterator vec_it = input.begin();for (; vec_it != input.end(); vec_it++){//将前k个元素插入集合if (leastNums.size()<k)leastNums.insert(*vec_it);else{//第一个元素是最大值multiset<int, greater<int> >::iterator greatest_it = leastNums.begin();//如果后续元素<第一个元素，删除第一个，加入当前元素if (*vec_it<*(leastNums.begin())){leastNums.erase(greatest_it);leastNums.insert(*vec_it);}}}return std::vector<int>(leastNums.begin(), leastNums.end());}};void show(vector<int> list){for (int i = 0; i < list.size(); i++){cout << list.at(i) << " ";}cout << endl;}/**算法之每日一题：最小的K个数输入n个整数，找出其中最小的K个数。例如输入4,5,1,6,2,7,3,8这8个数字，则最小的4个数字是1,2,3,4,。https://www.nowcoder.com/questionTerminal/6a296eb82cf844ca8539b57c23e6e9bf*/void main(){vector<int> list = { 4, 5, 1, 6, 2, 7, 3, 8 };show(list);Solution s;vector<int> result=s.GetLeastNumbers_Solution(list, 4);show(result);show(list);result = s.GetLeastNumbers_Solution_Partiton(list, 4);show(result);show(list);result = s.GetLeastNumbers_Solution_allsort(list, 4);show(result); show(list);result = s.GetLeastNumbers_Solution_headSort_custom(list, 4);show(result);show(list);result = s.GetLeastNumbers_Solution_heapSort(list, 4);show(result);system("pause");}
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