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Educational Codeforces Round 53 (Rated for Div. 2) C(二分)

C. Vasya and Robot

题解:考虑去根据左端点二分答案即可。

代码

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#include<bits/stdc++.h>
using namespace std;
const int N = 200100;
int U[N],R[N],L[N],D[N];
int n,x,y,sx,sy;
bool ok(int s,int t)
{
int ret = t - s + 1, tx = sx, ty = sy;
tx += L[t] - L[s - 1];
ty += D[t] - D[s - 1];
tx -= R[t] - R[s - 1];
ty -= U[t] - U[s - 1];
if(abs(x - tx) + abs(y - ty) > ret) return 0;
return (ret - abs(x - tx) - abs(y - ty)) % 2 == 0;
}

int main()
{
#ifndef ONLINE_JUDGE
freopen("input.in","r",stdin);
#endif
string s;
cin>>n>>s>>x>>y;
for(int i = 0; i < n; ++i) {
if(s[i] == 'L') sx--, L[i + 1] = 1;
if(s[i] == 'R') sx++, R[i + 1] = 1;
if(s[i] == 'U') sy++, U[i + 1] = 1;
if(s[i] == 'D') sy--, D[i + 1] = 1;
}
if(sx == x && sy == y) return cout<<0<<endl, 0;
for(int i = 1; i <= n; ++i) {
L[i] += L[i - 1];
R[i] += R[i - 1];
U[i] += U[i - 1];
D[i] += D[i - 1];
}
int ans = n + 1;
for(int i = 1, f = 0; i <= n; ++i, f = 0){
int l = i, r = n;
while(l <= r) {
int mid = l + (r - l) / 2;
if(!ok(i,mid)) {
l = mid + 1;
}else{
r = mid - 1;
f = 1;
}
}
if(f) ans = min(ans,r - i + 2);
}
if(ans == n + 1) ans = -1;
cout<<ans<<endl;
return 0;
}
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