POJ 3185

 1 #include <iostream>
 2 #include <cstdio>
 3 #include <cstring>
 4 #include <algorithm>
 5 
 6 using namespace std;
 7 
 8 int w[25];
 9 int flip[25];
10 
11 int get(int x) {
12     int c = w[x] + flip[x];
13     if(x - 1 >= 1) c += flip[x - 1];
14     if(x + 1 <= 20) c += flip[x + 1];
15     return c % 2;
16 }
17 
18 int cal() {
19         int res = 0;
20         for(int i = 2; i <= 20; ++i) {
21                 if(get(i - 1)) flip[i] = 1;
22         }
23 
24         if(get(20)) return -1;
25         for(int i = 1; i <= 20; ++i) res += flip[i];
26 
27         return res;
28 }
29 
30 void solve() {
31         int ans = -1;
32         for(int v = 0; v <= 1; ++v) {
33                 memset(flip,0,sizeof(flip));
34                 flip[1] = v;
35                 int num = cal();
36                 if(num >= 0 && (ans < 0 || ans > num)) {
37                         ans = num;
38                 }
39         }
40 
41         printf("%d\n",ans);
42 }
43 
44 int main()
45 {
46     //freopen("sw.in","r",stdin);
47     for(int i = 1; i <= 20; ++i) {
48             scanf("%d",&w[i]);
49     }
50 
51     solve();
52 
53     return 0;
54 }