So we can solve this question by sliding window method. We always keep a window that contains k girls' position and process the string char by char. If we encounter a boy, we do nothing; otherwise we move the window to contain the current girl and pop out the left most girl.

To solve the minimum swaps needed in the current window, one can find the median position of k girls in the window and calculate the distances from the other girls to the girl in the median position. For example, consider string = "B[GGGBBBBG]BBGGBGG", the median position in the current window is 2 or 3 (indexed from 0). Let the median position be 2, the total distances from all the other girls is 1 + 1 + 6 = 8. Then you minus the fixed adjust distance (1 + 1 + 2) = 4 and get the minimum number of swaps = 4. The fixed adjust distance here is because you move all girls into the median position, but the girls are sitting in a consecutive row instead of the same position.

Therefore we already have a O(kn)-time O(k)-space algorithm, k is for the time to solve the minimum number of swaps in the current window and n is for the time to scan the whole string.

Of course you can improve the algorithm to be O(n) time. This is because when the window is sliding to the next one, the median position can be tracked easily if the window is implemented by a linked list. Regarding updating the distances to the median position, let the left_distance be the total distances from the girls to the left of the median girl and similarly for the right_distance. Let m be the median position and m' be the new median position. Consider the window slides to the next girl at position j and pops the most left girl at position i. The the new left_distance = left_distance - (m - i) + (m' - m) * ((k - 1) / 2) and the new right_distance = right_distance + (j - m') - (m' - m) * (k / 2). Therefore we can calculate the minimum number of swaps in constant time.

Here is the code:

int minSwapNum(string& s, int k) {
list<int> window;
int i = 0;
while ((i < s.size()) && (window.size() < k)) {
if (s[i] == 'G') {
window.push_back(i);
}
i++;
}
//No enough girls
if (window.size() < k) {
return -1;
}
auto med_it = window.begin();
advance(med_it, (k - 1) / 2);
int left_dist = 0;
int right_dist = 0;
for (auto idx : window) {
if (idx < *med_it) {
left_dist += (*med_it - idx);
} else {
right_dist += (idx - *med_it);
}
}
int adjust_dist = (k % 2 == 0) ? k * k / 4 : (k * k - 1) / 4;
int min = left_dist + right_dist - adjust_dist;
for (; i<s.size(); i++) {
if (s[i] == 'G') {
int head = window.front();
int tail = i;
int old_med = *med_it;
window.pop_front();
window.push_back(tail);
med_it++;
left_dist = left_dist - (old_med - head) + (*med_it - old_med) * ((k - 1) / 2);
right_dist = right_dist + (tail - *med_it) - (*med_it - old_med) * (k / 2);
if (left_dist + right_dist - adjust_dist < min) {
min = left_dist + right_dist - adjust_dist;
}
}
}
return min;
}