这份题解的特点：

• 堆的实现是用迭代法写的。
• 用一个专门的数组记录第 k 个插入的数字当前的值。

#pragma GCC push_options
#pragma GCC optimize ("-Ofast")
#include <bits/stdc++.h>

constexpr int N = 100000;

struct MinHeap final {
    int arr[N];
    int len = 0;

    [[nodiscard]] constexpr int top() const noexcept
    {
        return arr[0];
    }

    constexpr void push(int newValue) noexcept
    {
        int i = len++;
        while (i) {
            int p = (i + 1) / 2 - 1;
            if (arr[p] <= newValue) break;
            arr[i] = arr[p];
            i = p;
        }
        arr[i] = newValue;
    }

    constexpr void pop() noexcept
    {
        int v = arr[--len], i = 0;
        while (true) {
            int j = 2 * i + 1;
            if (j >= len) break;
            if (j + 1 != len && arr[j + 1] < arr[j]) ++j;
            if (v <= arr[j]) break;
            arr[i] = arr[j];
            i = j;
        }
        arr[i] = v;
    }
} numbers, to_pop;

constexpr void flushHeapPops() noexcept
{
    while (to_pop.len != 0 && to_pop.top() == numbers.top()) {
        to_pop.pop();
        numbers.pop();
    }
}

struct BoundedArray final {
    int buffer[N];
    int len = 0;

    constexpr void push(int x) noexcept
    {
        buffer[len++] = x;
    }

    constexpr int& operator[](int offset) noexcept
    {
        return buffer[offset];
    }
} insert_sequence;

int main()
{
    char cmd[3];
    int n, k, x;
    scanf("%d", &n);
    for (int i = 0; i != n; ++i) {
        scanf("%s", cmd);
        if (cmd[0] == 'I') {
            scanf("%d", &x);
            numbers.push(x);
            insert_sequence.push(x);
        } else if (cmd[0] == 'P') {
            flushHeapPops();
            printf("%d\n", numbers.top());
        } else if (cmd[0] == 'C') {
            // Changing a value is equivalent to deleting the old and inserting the new.
            scanf("%d%d", &k, &x);
            to_pop.push(insert_sequence[k - 1]);
            numbers.push(x);
            insert_sequence[k - 1] = x;
        } else if (cmd[1] == 'M') {
            flushHeapPops();
            numbers.pop();
        } else {
            scanf("%d", &k);
            to_pop.push(insert_sequence[k - 1]);
        }
    }
}