[剑指offer] 29. 最小的K个数

题目描述

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,。

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思路：

排序后取前k，这里用堆排序

class Solution
{
  public:
    vector<int> GetLeastNumbers_Solution(vector<int> input, int k)
    {
        vector<int> res;
        if (input.empty() || k > input.size())
            return res;
        HeapSort(input);

        for (int i = 0; i < k; i++)
        {
            res.push_back(input[i]);
        }
        return res;
    }
    void PercDown(vector<int> &arr, int N, int i)
    {
        int child;
        int tempNode;
        // 2*i + 1 ： leftChild
        for (tempNode = arr[i]; (2 * i + 1) < N; i = child)
        {
            child = 2 * i + 1;
            if (child != N - 1 && arr[child + 1] > arr[child])
                child++;

            if (tempNode < arr[child])
                arr[i] = arr[child];
            else
                break;
        }
        arr[i] = tempNode;
    }

    void HeapSort(vector<int> &arr)
    {
        int len = arr.size();
        if (len == 0)
            return;
        for (int i = len / 2; i >= 0; i--)
            PercDown(arr, len, i); //biuld

        for (int i = len - 1; i > 0; i--)
        {
            swap(arr[0], arr[i]); //delete
            PercDown(arr, i, 0);
        }
    }
};