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 + 7;
const int inf = 0x3f3f3f3f;
const LL INF = 0x3f3f3f3f3f3f3f3f;
const int mod = 1e9 + 7;
const double eps = 1e-8;

int n, a[N], mx[N], mn[N];

bool check(int x) {
    if(a[1] > x) return false;
    mx[1] = mn[1] = a[1];
    for(int i = 2; i < n; i++) {
        if(x - a[i-1] < a[i]) return false;
        mx[i] = min(a[1]-mn[i-1], a[i]);
        if(x-a[i-1]-a[1]+mx[i-1] >= a[i]) mn[i] = 0;
        else mn[i] = a[i] - (x-a[i-1]-a[1]+mx[i-1]);
    }
    int cnt = a[1] + a[n-1] - mx[n-1];
    return x - cnt >= a[n];
}

int main() {
    scanf("%d", &n);
    for(int i = 1; i <= n; i++)
        scanf("%d", &a[i]);
    if(n == 1) printf("%d\n", a[1]);
    else if(n == 2) printf("%d\n", a[1]+a[2]);
    else {
        int l = 1, r = 300000, ans = 300000;
        while(l <= r) {
            int mid = l + r >> 1;
            if(check(mid)) r = mid - 1, ans = mid;
            else l = mid + 1;
        }
        printf("%d\n", ans);
    }
    return 0;
}

/*
*/