Plants vs. Zombies 题解(二分+贪心)

题目链接

题目思路

肯定是二分

问题是如何check才是最优解

就是每次都是走相邻的位置

例如x没走完，那就一直循环走\(x,x+1\)这俩个位置

代码

#include<bits/stdc++.h>
#define fi first
#define se second
#define debug cout<<"I AM HERE"<<endl;
using namespace std;
typedef long long ll;
const int maxn=2e5+5,inf=0x3f3f3f3f,mod=1e9+7;
const double eps=1e-6;
int n;
ll m;
ll a[maxn];
ll b[maxn];
ll check(ll x){
    for(int i=1;i<=n;i++){
        b[i]=(x+a[i]-1)/a[i];
    }
    ll ans=0;
    int now=0;
    for(int i=1;i<=n;i++){
        if(b[i]==0) continue;
        ans+=i-now;
        ans+=2*(b[i]-1);
        b[i+1]-=(b[i]-1);
        b[i+1]=max(b[i+1],0ll);
        now=i;
    }
    return ans;
}
signed main(){
    int _;scanf("%d",&_);
    while(_--){
        scanf("%d%lld",&n,&m);
        for(int i=1;i<=n;i++){
            scanf("%lld",&a[i]);
        }
        ll l=1,r=(1e18)/n,ans=0;
        while(l<=r){
            ll mid=(l+r)/2;
            if(check(mid)<=m){
                ans=mid;
                l=mid+1;
            }else{
                r=mid-1;
            }
        }
        printf("%lld\n",ans);
    }
    return 0;
}