CF 797, got stuck on E and had to leave, went pretty well

codeforces/rounds/797/d.cpp

@@ -0,0 +1,68 @@
+#include <bits/stdc++.h>
+
+using namespace std;
+
+void test_case(int tc){
+    int n, k;
+    cin >> n >> k;
+
+    vector<bool> strip;
+
+    int blacks = 0;
+    int whites = 0;
+    for(int i = 0; i < k; ++i){
+        char c;
+        cin >> c;
+
+        if(c == 'B'){
+            blacks++;
+            strip.push_back(true);
+        } else{
+            whites++;
+            strip.push_back(false);
+        }
+    }
+
+    int min_whites = whites;
+
+    for(int i = 0; i < n - k; ++i){
+        char c;
+        cin >> c;
+
+        if(strip[i]){
+            blacks--;
+        } else {
+            whites--;
+        }
+
+        if(c == 'B'){
+            blacks++;
+            strip.push_back(true);
+        } else {
+            whites++;
+            strip.push_back(false);
+        }
+
+        min_whites = min(min_whites, whites);
+    }
+
+    cout << min_whites << '\n';
+}
+
+
+
+int main(){
+    ios::sync_with_stdio(0);
+    cin.tie(0);
+
+    int t;
+    cin >> t;
+
+    for(int tc = 1; tc <= t; ++tc){
+        test_case(tc);
+    }
+
+    cout << flush;
+
+    return 0;
+}