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Luogu/P_1015_NOIP_1999_普及组_回文数.cpp

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+#include <bits/stdc++.h>
+#define int long long
+#define endl "\n"
+#define IMX LONG_LONG_MAX
+#define IMN LONG_LONG_MIN
+#define debug$ if (dev)
+using namespace std;
+
+/* toothless. #17 */
+
+const int N = 1e7 + 10;
+const int M = 2e3 + 5;
+const bool dev = false;
+
+char s[M];
+int a[M], top;
+bool check()
+{
+    int t = top / 2;
+    for (int i = 0; i <= t; ++i)
+        if (a[i] != a[top - i])
+            return true;
+    return false;
+}
+
+signed main()
+{
+    ios::sync_with_stdio(0);
+    cin.tie(0);
+    cout.tie(0);
+    int n, step = 0;
+    cin >> n >> s;
+    top = strlen(s) - 1;
+    for (int i = 0; i <= top; ++i)
+        if (s[i] >= '0' && s[i] <= '9')
+            a[top - i] = s[i] - '0';
+        else
+            a[top - i] = s[i] - 55;
+    while (check()) {
+        if (step > 30) {
+            cout << "Impossible!";
+            return 0;
+        }
+        for (int i = 0; i <= top; ++i)
+            if (top - i >= i)
+                a[i] += a[top - i];
+            else
+                a[i] = a[top - i];
+        for (int i = 0; i <= top; ++i)
+            if (a[i] >= n)
+                a[i + 1]++, a[i] -= n;
+        if (a[top + 1])
+            top++;
+        step++;
+    }
+    cout << "STEP=" << step;
+    return 0;
+}