重复尝试后 $\color{green}AC$ 了，算法大概是没问题，不知道为什么会引入某种不确定性因素导致 $\color{purple}RE$ 。有时甚至直接 $\color{orange}CE$ 。

#include <iostream>
using namespace std;
typedef long long ll;

ll l, *stones;
int n, most;

bool canDo(ll m)
{
    int removed = 0;
    int before = 0;
    for (int i = 0; i < n; i++)
    {
        if (stones[i] - before >= m)
            before = stones[i];
        else
            removed++;
    }
    if (l - before < m)
        removed++;
    // cout << m << ':' << removed << endl;
    return removed <= most;
}

int main()
{
    ios::sync_with_stdio(false);
    cin >> l >> n >> most;
    stones = new ll[n];
    for (int i = 0; i < n; i++)
        cin >> stones[i];

    ll left = 1;
    ll right = l;
    while (left < right)
    {
        ll mid = (left + right) / 2;
        if (canDo(mid))
            left = mid + 1;
        else
            right = mid - 1;
    }
    if (canDo(left))
        cout << left;
    else
        cout << left - 1;
    return 0;
}