这里放上AC代码，解析可以看老师的直播

#include<bits/stdc++.h>
#define ll long long
using namespace std;
const ll N=5e4+5;
ll L,n,m,a[N];
bool check(ll x){
	ll cnt=0,last=0;
	for(ll i=1;i<=n;i++)
		if(a[i]-a[last]>=x&&L-a[i]>=x)
			last=i;
		else
			cnt++;
	return cnt<=m;
}
ll bf(){
	ll l=1,mid,r=L;
	while(l<=r){
		mid=l+r>>1;
		if(check(mid))
			l=mid+1;
		else
			r=mid-1;
	}
	return r;
}
int main(){
	scanf("%lld%lld%lld",&L,&n,&m);
	for(ll i=1;i<=n;i++)
		scanf("%lld",&a[i]);
	printf("%lld",bf());
	return 0;
}