建堆之后堆顶是几

把 17 24 15 13 23 建成大顶堆。

运行下面这段程序：

#include <algorithm>
#include <iostream>
#include <map>
#include <string>
#include <utility>
#include <vector>
using namespace std;

void sift_down(vector<int>& h, int i, int n) {
    while (true) {
        int big = i;
        int l = 2 * i + 1, r = 2 * i + 2;
        if (l < n && h[l] > h[big]) big = l;
        if (r < n && h[r] > h[big]) big = r;
        if (big == i) return;
        swap(h[i], h[big]);
        i = big;
    }
}

vector<int> hsort(vector<int> h) {
    int n = (int)h.size();
    for (int i = n / 2 - 1; i >= 0; i--) sift_down(h, i, n);
    for (int end = n - 1; end > 0; end--) {
        swap(h[0], h[end]);
        sift_down(h, 0, end);
    }
    return h;
}

int main() {
    vector<int> h = {17, 24, 15, 13, 23};
    int n = (int)h.size();
    for (int i = n / 2 - 1; i >= 0; i--) sift_down(h, i, n);
    cout << h[0] << endl;
}

（本题用 g++ -std=c++17 -O0 编译。）