The sword refers to the minimum k number of the twenty-ninth question of the offer

Topic description

Enter n integers and find the smallest K number among them. For example, if you enter 8 numbers 4,5,1,6,2,7,3,8, the smallest 4 numbers are 1,2,3,4,.

topic analysis

The topic is changed, it is simple and easy to do, just go to the code. Can library function sort, quicksort idea and heap sort

class Solution {
public:
    vector<int> GetLeastNumbers_Solution(vector<int> input, int k) {
        int size=input.size();
        vector<int> ret;
        if(input.empty()||k>input.size()) return ret;
        sort(input.begin(),input.end());
        for(int i=0;i<k;i++){
            ret.push_back(input[i]);
        }
        return ret;
    }
};

Quick row

class Solution {
public:
    void swap(int &fir,int &sec)
    {
        int temp = fir;
        fir = sec;
        sec = temp;
    }
     
    int getPartition(vector<int> &input,int start,int end)
    {
        if(input.empty() || start>end) return -1;
        int temp = input[end];
        int j = start - 1;
        for(int i=start;i<end;++i)
        {
            if(input[i]<=temp)
            {
                ++j;
                if(i!=j) swap(input[i],input[j]);                   
            }
        }
        swap(input[j+1],input[end]);
        return (j+1);
    }
         
    vector<int> GetLeastNumbers_Solution(vector<int> input, int k)
    {
        vector<int> result;       
        if(input.empty() || k>input.size() || k<=0) return result;
         
        int start = 0;
        int end = input.size()-1;
        int index = getPartition(input,start,end);
         
        while(index != (k-1))
        {
            if(index > (k-1))
            {
                end = index - 1;
                index = getPartition(input,start,end);
            }
            else
            {
                start = index + 1;
                index = getPartition(input,start,end);
            }
        }
         
        for(int i=0;i<k;++i)
        {
            result.push_back(input[i]);
        }
         
        return result;
    }
};

heap sort

class Solution {
public:
    vector<int> GetLeastNumbers_Solution(vector<int> input, int k)
    {
        vector<int> result;
        int len = input.size();
        if(input.empty() || k<=0 || len < k) return result;
         
        multiset<int, greater<int> > leastNumbers; // 从大到小排序
        multiset<int, greater<int> >::iterator iterGreater; // 定义迭代器
         
        vector<int>::iterator iter = input.begin();
        for(; iter != input.end(); ++iter)
        {
            // 将前k个数直接插入进multiset中，注意是小于K
            if(leastNumbers.size() < k)
            {
                leastNumbers.insert(*iter);
            }
            else
            {
                // 因为设置的从大到小排序，故multiset中第一个位置的元素即为最大值
                iterGreater = leastNumbers.begin();
                 
                // 如果input中当前元素比multiset中最大元素小，则替换；即保持multiset中这k个元素是最小的。
                if(*iter < *(leastNumbers.begin()))
                {
                    // 替换掉当前最大值
                    leastNumbers.erase(iterGreater);
                    leastNumbers.insert(*iter);
                }
            }
        }
         
        for(iterGreater = leastNumbers.begin();iterGreater!=leastNumbers.end();++iterGreater)
        {
            result.push_back(*iterGreater); // 将multiset中这k个元素输出
        }
         
        return result;
    }
};