[AcWing 53] 最小的 k 个数

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堆排序

点击查看代码

class Solution {
public:
    vector<int> getLeastNumbers_Solution(vector<int> input, int k) {
        priority_queue<int> heap;
        for (auto x : input) {
            heap.push(x);
            if (heap.size() > k)    heap.pop();
        }
        vector<int> res;
        while (heap.size()) res.push_back(heap.top()), heap.pop();
        reverse(res.begin(), res.end());
        return res;
    }
};

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1. 维护一个大根堆 heap，每次把 vector 中的元素放入到 heap 中，当 heap 中元素的个数大于 k 时，使用 heap.pop() 移除 heap 中的最大值；

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快速选择

点击查看代码

class Solution {
public:
    vector<int> getLeastNumbers_Solution(vector<int> input, int k) {
        vector<int> res;
        for (int i = 1; i <= k; i ++) {
            res.push_back(quick_select(input, 0, input.size() - 1, i));
        }
        return res;
    }
    int quick_select(vector<int> q, int l, int r, int k)
    {
        if (l >= r) return q[l];
        int i = l - 1, j = r + 1, x = q[l + r >> 1];
        while (i < j) {
            do i ++; while (q[i] < x);
            do j --; while (q[j] > x);
            if (i < j)  swap(q[i], q[j]);
        }
        if (k <= j - l + 1) return quick_select(q, l, j, k);
        else    return quick_select(q, j + 1, r, k - (j - l + 1));
    }
};

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1. quick_select 模板;