#965. [雅礼20191012]交错的字符串（string）

https://sjzezoj.com/submission/55115

S2OJ/965/965.cpp

@@ -0,0 +1,69 @@
+#include <iostream>
+#include <string>
+#include <unordered_map>
+
+using std::cin;
+using std::cout;
+const char endl = '\n';
+
+const int N = (18 << 1) + 5;
+
+int n;
+long long ans;
+bool vis[N];
+std::string s;
+std::unordered_map<std::string, int> map;
+
+void dfs1(int x) {
+    if (x == n + 1) {
+        std::string s1, s2;
+
+        for (int i = 1; i <= n; i++) {
+            (vis[i] ? s2 : s1).push_back(s[i]);
+        }
+
+        map[s1 + ' ' + s2]++;
+
+        return;
+    }
+
+    dfs1(x + 1);
+    vis[x] = true;
+    dfs1(x + 1);
+    vis[x] = false;
+}
+
+void dfs2(int x) {
+    if (x == n * 2 + 1) {
+        std::string s1, s2;
+
+        for (int i = n + n; i > n; i--) {
+            (vis[i] ? s2 : s1).push_back(s[i]);
+        }
+
+        ans += map[s1 + ' ' + s2];
+
+        return;
+    }
+
+    dfs2(x + 1);
+    vis[x] = true;
+    dfs2(x + 1);
+    vis[x] = false;
+}
+
+int main() {
+    std::ios::sync_with_stdio(false);
+    cin.tie(nullptr);
+
+    cin >> n >> s;
+
+    s = ' ' + s;
+
+    dfs1(1);
+    dfs2(n + 1);
+
+    cout << (ans >> 1) << endl;
+
+    return 0;
+}