BZOJ1863 [Zjoi2006]trouble 皇帝的烦恼

 1 /**************************************************************
 2     Problem: 1863
 3     User: rausen
 4     Language: C++
 5     Result: Accepted
 6     Time:44 ms
 7     Memory:1976 kb
 8 ****************************************************************/
 9  
10 #include <cstdio>
11 #include <algorithm>
12  
13 using namespace std;
14 const int N = 100005;
15 int n, ans;
16 int v[N], a[N], b[N];
17  
18 inline int read(){
19     int x = 0;
20     char ch = getchar();
21     while (ch < ‘0‘ || ch > ‘9‘)
22         ch = getchar();
23     while (ch >= ‘0‘ && ch <= ‘9‘){
24         x = x * 10 + ch - ‘0‘;
25         ch = getchar();
26     }
27     return x;
28 }
29  
30 bool check(const int x){
31     int i;
32     a[1] = b[1] = v[1];
33     for (i = 2; i <= n; ++i){
34         a[i] = max(0, v[1] + v[i] + v[i - 1] - b[i - 1] - x);
35         b[i] = min(v[i], v[1] - a[i - 1]);
36     }
37     return !a[n];
38 }
39  
40 int main(){
41     int i, l, r, mid;
42     n = read();
43     for (i = 1; i <= n; ++i)
44         ans = max((v[i] = read()) + v[i - 1], ans);
45     l = ans - 1, r = ans * 2;
46     while (l + 1 < r){
47         mid = l + r >> 1;
48         if (check(mid)) r = mid;
49         else l = mid;
50     }
51     printf("%d\n", r);
52     return 0;
53 }