A.一定是前半段白后半段黑，枚举分界点预处理前后缀和。
1 #include<cstdio> 2 #include<cstring> 3 #include<algorithm> 4 #include<iostream> 5 #define rep(i,l,r) for (int i=(l); i<=(r); i++) 6 typedef long long ll; 7 using namespace std; 8 9 const int N=200010; 10 char s[N]; 11 int n,ans,pre[N],suf[N]; 12 13 int main(){ 14 freopen("a.in","r",stdin); 15 freopen("a.out","w",stdout); 16 scanf("%d%s",&n,s+1); ans=n+1; 17 rep(i,1,n) pre[i]=pre[i-1]+(s[i]=='#'); 18 for (int i=n; i; i--) suf[i]=suf[i+1]+(s[i]=='.'); 19 rep(i,0,n) ans=min(ans,pre[i]+suf[i+1]); 20 printf("%d\n",ans); 21 return 0; 22 }