feat(cses): a few problems

cses/missing-number/.clang-format

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+BasedOnStyle: Google
+AllowShortBlocksOnASingleLine: false
+AllowShortCaseLabelsOnASingleLine: false
+AllowShortCompoundRequirementOnASingleLine: false
+AllowShortEnumsOnASingleLine: false
+AllowShortFunctionsOnASingleLine: false
+AllowShortIfStatementsOnASingleLine: false
+AllowShortLambdasOnASingleLine: false
+AllowShortLoopsOnASingleLine: false

cses/missing-number/.clangd

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+CompileFlags:
+	Add:
+		- -Wall
+		- -Wextra
+		- -Wpedantic
+		- -Wshadow
+		- -DLOCAL
+		- -Wno-unknown-pragmas

cses/missing-number/compile_flags.txt

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+-Wall
+-Wextra
+-Wpedantic
+-Wshadow
+-DLOCAL
+-std=c++20

cses/missing-number/solution.cc

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+#include <bits/stdc++.h>  // {{{
+
+// https://codeforces.com/blog/entry/96344
+
+#pragma GCC optimize("O2,unroll-loops")
+#pragma GCC target("avx2,bmi,bmi2,lzcnt,popcnt")
+
+using namespace std;
+
+template <typename T>
+constexpr T MIN = std::numeric_limits<T>::min();
+
+template <typename T>
+constexpr T MAX = std::numeric_limits<T>::max();
+
+template <typename T>
+[[nodiscard]] static T sc(auto&& x) {
+  return static_cast<T>(x);
+}
+
+template <typename T>
+[[nodiscard]] static T sz(auto&& x) {
+  return static_cast<T>(x.size());
+}
+
+using ll = long long;
+using ld = long double;
+template <typename T>
+using vec = std::vector<T>;
+template <typename T, size_t N>
+using arr = std::array<T, N>;
+
+#define ff first
+#define ss second
+#define eb emplace_back
+#define pb push_back
+#define all(x) (x).begin(), (x).end()
+#define rall(x) (x).rbegin(), (x).rend()
+//  }}}
+
+void solve() {
+  int n;
+  cin >> n;
+  vec<bool> seen(n + 1, false);
+  while (--n) {
+    int x;
+    cin >> x;
+    seen[x] = true;
+  }
+  for (int i = 1; i < sz<int>(seen); ++i) {
+    if (!seen[i]) {
+      cout << i << endl;
+      break;
+    }
+  }
+}
+
+int main() {  // {{{
+  cin.tie(nullptr)->sync_with_stdio(false);
+
+  solve();
+
+  return 0;
+}
+// }}}

cses/missing-number/solution.in

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+5
+2 3 1 5

cses/missing-number/solution.out

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+4
+
+[code]: 0
+[time]: 3.88908 ms