开始计概A下半学期。

cs101/2025mid/exam_2025mid_06_perm.cpp

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+#include <iostream>
+#include <vector>
+
+bool validate_arithmetic(const std::vector<bool>& bucket) {
+    if (bucket.size() <= 1) { return false; }
+    int previous_idx = -1, distance = -1;
+    for (int i = 0; i < bucket.size(); i++) {
+        if (!bucket[i]) { continue; }
+        if (previous_idx == -1) {
+            previous_idx = i;
+            continue;
+        }
+        if (distance == -1) {
+            distance = i - previous_idx;
+        }
+        if (i - previous_idx != distance) {
+            return false;
+        }
+        previous_idx = i;
+    }
+    return true;
+}
+
+int clac_ans(const std::vector<int>& nums) {
+    if (nums.size() == 1) { return 0; }
+    std::vector<bool> bucket_begin(nums.size(), false);
+    std::vector<bool> bucket_end(nums.size(), true);
+
+    int ans = 0;
+    for (int i = 0; i < nums.size(); i++) {
+        bucket_begin[nums[i]] = true;
+        bucket_end[nums[i]] = false;
+        if (validate_arithmetic(bucket_begin) && validate_arithmetic(bucket_end)) {
+            ans++;
+            std::clog << i << std::endl;
+        }
+    }
+    return ans - 1;
+}
+
+int main() {
+    std::ios::sync_with_stdio(false);
+    std::cin.tie(nullptr);
+
+    int n;
+    std::cin >> n;
+    while (std::cin >> n) {
+        std::vector<int> nums(n);
+        for (int i = 0; i < n; i++) { std::cin >> nums[i], nums[i]--; }
+        std::cout << clac_ans(nums) << std::endl;
+    }
+
+    return 0;
+}