fixed a severe index error

test/t1_mk.cpp

@@ -0,0 +1,64 @@
+#include <bits/stdc++.h>
+using namespace std;
+const unsigned int RndSeed = random_device{}();
+mt19937 rnd(RndSeed);
+int rnd_less(int n) { return rnd() % n; }
+const int kMaxN = 5e4 + 10;
+int fa[kMaxN];
+inline int ff(int u) {
+  int x = u, y;
+  while (fa[u] != u) u = fa[u];
+  while (x != u) {
+    y = fa[x];
+    fa[x] = u;
+    x = y;
+  }
+  return u;
+}
+int main(int argc, char *argv[]) {
+  cerr << "[ Data Generator: Seed = " << RndSeed << " ]" << endl;
+  FILE *fout = fopen("GeneratorSeed.txt", "a");
+  fprintf(fout, "Seed = %u\n", RndSeed);
+  fclose(fout);
+  // ======================================
+  int n = 1000;
+  int total_keys = 300;
+  set<string> keys_set;
+  for (int i = 0; i < total_keys; i++) {
+    string key = "#" + to_string(rnd_less(1000000)) + "#";
+    keys_set.insert(key);
+  }
+  vector<string> keys_vec;
+  unordered_map<string, vector<int>> mp;
+  for (auto &key : keys_set) {
+    keys_vec.push_back(key);
+  }
+  cout << n << endl;
+  for (int i = 0; i < n; i++) {
+    int tmp = rnd() % 10;
+    if (tmp <= 4) {
+      string key = keys_vec[rnd_less(keys_vec.size())];
+      int val = rnd_less(1000000);
+      cout << "insert " << key << " " << val << "\n";
+      // 往 mp[key] 有序地插入
+      auto it = lower_bound(mp[key].begin(), mp[key].end(), val);
+      if (it == mp[key].end() || *it != val) mp[key].insert(it, val);
+    } else if (tmp <= 6) {
+      string key = keys_vec[rnd_less(keys_vec.size())];
+      int val = rnd_less(1000000);
+      if (rnd() % 2 == 0 && mp[key].size() > 0) {
+        // 选择一个有意义的删除项
+        val = mp[key][rnd_less(mp[key].size())];
+      }
+      cout << "delete " << key << " " << val << "\n";
+      // 从 mp[key] 中删除 val
+      auto it = lower_bound(mp[key].begin(), mp[key].end(), val);
+      if (it != mp[key].end() && *it == val) mp[key].erase(it);
+      if (mp[key].empty()) mp.erase(key);
+    } else {
+      string key = keys_vec[rnd_less(keys_vec.size())];
+      cout << "find " << key << "\n";
+    }
+  }
+  return 0;
+}