IOI 2017 Practice Contest mountains

Mountains

题面

题意：

　　　选最多的点使得两两看不见。

分析：

　　分治，solve(l,r)为区间[l,r]的答案。那么如果不选最高点，分治两边即可，选了最高点，那么在最高点看不见的区间里分治。

代码：

 1 #include"mountains.h"
 2 #include
 3 using namespace std;
 4  
 5 const int N = 2010;
 6 int y[N];
 7 int f[N][N];
 8  
 9 bool Judge(int a,int b,int c) { // a see c ?
10     // ((y[a] - y[b]) / (a - b)) > ((y[a] - y[c]) / (a - c));
11     return 1ll * (y[a] - y[b]) * (a - c) > 1ll * (y[a] - y[c]) * (a - b);
12 }
13  
14 int solve(int l,int r) {
15     if (f[l][r]) return f[l][r];
16     if (l > r) return 0;
17     if (l == r) return 1;
18     int pos = -1, mx = -1;
19     for (int i=l; i<=r; ++i) if (y[i] > mx) pos = i, mx = y[i];
20     int ans1 = solve(l, pos - 1) + solve(pos + 1, r); // select max_height_node
21     
22     int ans2 = 0; // don't select this node
23     for (int i=pos-1,j; i>=l; i=j) {
24         for (j=i-1; j>=l; --j) {  // i can see pos
25             if (Judge(j, i, pos)) { // j can't see pos
26                 if (j == l) ans2 += solve(l, i - 1); 
27                 continue;
28             }
29             if (j + 1 <= i - 1) ans2 += solve(j + 1, i - 1); // this Section can't see pos
30             break;
31         }
32     }
33     for (int i=pos+1,j; i<=r; i=j) {
34         for (j=i+1; j<=r; ++j) {
35             if (Judge(pos, i, j)) {
36                 if (j == r) ans2 += solve(i + 1, r);
37                 continue;
38             }
39             if (i + 1 <= j - 1) ans2 += solve(i + 1, j - 1);
40             break;
41         }
42     } 
43     return f[l][r] = max(ans1, ans2 + 1);
44 }
45  
46 int maximum_deevs(vector<int> A) {
47     int n = A.size();
48     for (int i=1; i<=n; ++i) y[i] = A[i - 1];
49     return solve(1, n);
50 }

 

转载于:https://www.cnblogs.com/mjtcn/p/9600106.html