Codeforces対策（随時更新）

■マス

十字架があるかないか→https://babcs2035.hateblo.jp/entry/2019/06/12/063912

■数列

maximum possible number of elements divisible by 3 （1位の人）⇒

int main(){
	cin>>n>>k;
	REP(i,n)cin>>h[i];
	sort(h,h+n);
	ll ans = 1ll<<60;
	REP(i,n)if(i+k<=n)CHMIN(ans,h[i+k-1]-h[i]);
	cout<<ans<<endl;
	return 0;
}

全項の積の最大値(2位の人)

int main() {

	ios_base::sync_with_stdio(0);
	cin.tie(0);
	int n;
	cin >> n;
	int a[n];
	for(int i = 0; i < n; ++i) {
		cin >> a[i];
		if(a[i] >= 0)
			a[i] = -a[i] - 1;
	}
	if(n % 2 == 1) {
		int mn = -1, mxi;
		for(int i = 0; i < n; ++i)
			if(mn > a[i]) {
				mn = a[i];
				mxi = i;
			}
		if(mn == -1) 
			a[0] = 0;
		else a[mxi] = -a[mxi] - 1;
	}
	for(int i = 0; i < n; ++i)
		cout << a[i] << " ";

}

■構文解析（1位の人）

#include <iostream>
#include <vector>
using namespace std;
using ll = long long;
const ll MAXV = (1ll<<32);

int main() {
	ios_base::sync_with_stdio(false);
	cin.tie(0);

	int n;
	cin >> n;

	ll res = 0;
	vector<ll> stack = {1};
	for (int i = 0; i < n; ++i) {
		string str;
		cin >> str;
		if (str == "add") {
			res += stack.back();
		} else if (str == "for") {
			ll k;
			cin >> k;
			stack.push_back(min(MAXV, k*stack.back()));
		} else if (str == "end") {
			stack.pop_back();
		}
	}
	if (res >= MAXV) cout << "OVERFLOW!!!\n";
	else cout << res << '\n';
}

■文字列（2位の人）

#include <bits/stdc++.h>
using namespace std;

int main() {
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);
	int n;
	cin>>n;
	stringstream ss;
	for (int _ = 0; _ < n; _++) {
	    string a,b; cin>>a>>b;
	    int i = 0, j = 0;
	    bool success = 1;
	    while (i<a.length() && j<b.length()) {
	        int ii = i;
	        while (ii<a.length() && a[ii]==a[i]) ii++;
	        int jj = j;
	        while (jj<b.length() && b[jj]==b[j]) jj++;
	        if (a[i] != b[j]) success = 0;
	        else if (ii-i>jj-j) success = 0;
	        i = ii;
	        j = jj;
	    }
	    if (i!=a.length() || j!=b.length()) success = 0;
	    ss << (success?"YES":"NO") << endl;
	}
	cout << ss.str();
	return 0;
}