bzoj 1863 二分+dp check

思路：二分之后用dp去check就好啦。

#include<bits/stdc++.h>
#define LL long long
#define fi first
#define se second
#define mk make_pair
#define PII pair<int, int>
#define PLI pair<LL, int>
#define ull unsigned long long
using namespace std;
 
const int N = 1e5 + ;
const int inf = 0x3f3f3f3f;
const LL INF = 0x3f3f3f3f3f3f3f3f;
const int mod = 1e9 + ;
const double eps = 1e-;
 
int n, a[N], mx[N], mn[N];
 
bool check(int x) {
    if(a[] > x) return false;
    mx[] = mn[] = a[];
    for(int i = ; i < n; i++) {
        if(x - a[i-] < a[i]) return false;
        mx[i] = min(a[]-mn[i-], a[i]);
        if(x-a[i-]-a[]+mx[i-] >= a[i]) mn[i] = ;
        else mn[i] = a[i] - (x-a[i-]-a[]+mx[i-]);
    }
    int cnt = a[] + a[n-] - mx[n-];
    return x - cnt >= a[n];
}
 
int main() {
    scanf("%d", &n);
    for(int i = ; i <= n; i++)
        scanf("%d", &a[i]);
    if(n == ) printf("%d\n", a[]);
    else if(n == ) printf("%d\n", a[]+a[]);
    else {
        int l = , r = , ans = ;
        while(l <= r) {
            int mid = l + r >> ;
            if(check(mid)) r = mid - , ans = mid;
            else l = mid + ;
        }
        printf("%d\n", ans);
    }
    return ;
}
 
/*
*/