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,。

方法一：基于Partition函数的思路 时间复杂度是O(n) 需要修改输入数组  不适用于海量数据

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int Partition(vector<int> &input, int start, int end)
{
int i = start;
int temp = input[start], j = end;
if (start < end)
{
while (i < j)
{
while (i < j && input[j] > temp) j--;
if (i < j)
{
input[i++] = input[j];
}
while (i < j && input[i] < temp) i++;
if (i < j)
{
input[j--] = input[i];
}
}
input[i] = temp;
}
return i;
}
vector<int> GetLeastNumbers_Solution(vector<int> &input, int k) {
vector<int> res;
if (input.size() == 0 || input.size() < k || k <= 0)
return res;
int n = input.size();
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++)
res.push_back(input[i]);
return res;
}

void main()
{
    vector<int> input = { 4, 5, 1, 6, 2, 7, 3, 8 };
    vector<int> GetLeastNumbers_Solution_ = GetLeastNumbers_Solution(input, 4);
    return;
}

方法二：基于堆或红黑树的思路  时间复杂度是O(n*logK) 不需要修改输入数组  适用于海量数据

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typedef multiset<int, greater<int> > intSet;
typedef multiset<int, greater<int> >::iterator setIterator;
vector<int> GetLeastNumbers_Solution(vector<int> &input, int k)
{
    intSet lest;
    vector<int> res;
    if (input.size() == 0 || input.size() < k || k <= 0)
        return res;
    lest.clear();
    vector<int>::const_iterator iter = input.begin();
    for (; iter != input.end(); ++iter)
    {
        if (lest.size() < k)
        {
            lest.insert(*iter);
        }
        else
        {
            setIterator iterGreatest = lest.begin();
            if (*iter < * (lest.begin()))
            {
                lest.erase(iterGreatest);
                lest.insert(*iter);
            }
        }
    }
    for (auto it = lest.begin(); it != lest.end(); it++)
    {
        res.push_back(*it);
    }
    return res;
}

void main()
{
vector<int> input = { 4,5,1,6,2,7,3,8 };
vector<int> GetLeastNumbers_Solution_ = GetLeastNumbers_Solution(input, 1);
return;

}