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codeforces/prependappend/main.cpp

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+#include <iostream>
+#include <deque>
+using namespace std;
+
+void solve() {
+    int n;
+    cin >> n;
+    deque<int> string(n);
+    for(int i = 0; i < n; i++) {
+        char c;
+        cin >> c;
+        string[i] = c-48;
+    }
+
+    int count = 0;
+    while(string.size() > 1 && string.back() != string.front()) {
+        count++;
+        string.pop_back();
+        string.pop_front();
+    }
+    cout << n-2*count << endl;
+}
+
+int main() {
+    int t;
+    cin >> t;
+    for(int i = 0; i < t; i++) solve();
+}