This submission is migrated from previous version of oj.uz, which used different machine for grading. This submission may have different result if resubmitted.
#include <bits/stdc++.h>
using namespace std;
struct Fenwick {
int sz;
vector<int> f;
void add(int i, int val) {
for (++i; i <= sz; i += i & -i) {
f[i] += val;
}
}
int get(int i) {
int res = 0;
for (; i > 0; i -= i & -i) {
res += f[i];
}
return res;
}
Fenwick(int sz_) : sz(sz_) {
f.resize(sz + 1, 0);
}
};
int main() {
ios::sync_with_stdio(false);
cin.tie(0);
int n;
string s;
cin >> n >> s;
vector<int> black, white;
for (int i = 0; i < 2 * n; ++i) {
if (s[i] == 'W') {
white.push_back(i);
} else {
black.push_back(i);
}
}
sort(black.begin(), black.end());
sort(white.begin(), white.end());
auto get = [&](int k) {
k %= n;
vector<vector<int>> start(2 * n + 1), finish(2 * n + 1);
Fenwick fenw(2 * n);
vector<pair<int, int>> segments;
for (int i = 0; i < n; ++i) {
int x = black[i], y = white[(i + k) % n];
segments.push_back({min(x, y), max(x, y)});
}
for (int i = 0; i < n; ++i) {
start[segments[i].first + 1].push_back(i);
finish[segments[i].second].push_back(i);
}
long long res = 0;
for (int i = 0; i < 2 * n; ++i) {
for (int j : start[i]) {
fenw.add(segments[j].first, 1);
}
for (int j : finish[i]) {
fenw.add(segments[j].first, -1);
}
for (int j : finish[i]) {
res += fenw.get(2 * n) - fenw.get(segments[j].first + 1);
}
}
return res;
};
if (n == 1) {
cout << get(0) << endl;
return 0;
}
if (get(0) < get(1)) {
reverse(white.begin(), white.end());
reverse(black.begin(), black.end());
}
assert(get(0) > get(1));
long long g0 = get(0);
int l = 0, r = n;
while (r - l > 1) {
int m = (l + r) / 2;
if (get(m) < g0) {
l = m;
} else {
r = m;
}
}
r = n;
while (r - l > 5) {
int m = (l + r) / 2;
if (get(m) < get(m + 1)) {
l = m;
} else {
r = m + 1;
}
}
long long ans = 0;
for (int k = l; k <= r; ++k) {
ans = max(ans, get(k));
}
cout << ans << endl;
return 0;
}
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