...

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
 
const ll M = 1e9+7;
const ll N = 2e6+7;
ll fact[N], invFact[N];
 
ll bigMod(ll base, ll power) {
    if (!power) return 1;
    ll result = bigMod(base, power/2);
    result = (result * result) % M;
    if (power & 1) result = (result * base) % M;
    return result;
}
 
signed main()
{
    ios_base::sync_with_stdio(0); cin.tie(0);
    ll a, b, p, x; cin >> a >> b >> p >> x;
 
    fact[0] = 1;
    for (ll i = 1; i < N; i++) fact[i] = (fact[i-1] * i) % M;
    
    invFact[N-1] = bigMod(fact[N-1], M-2);
    for (ll i = N-2; i >= 0; i--) invFact[i] = (invFact[i+1] * (i+1)) % M;
 
    ll ans = 0;
    for (ll i = x; i <= min(p, a); i++) {
        ll firstTeam = (fact[a] * ((invFact[i]*invFact[a-i]) % M)) % M; // aCi
        if (b-p+i < 0) continue;
        ll secondTeam = (fact[b] * ((invFact[p-i]*invFact[b-p+i]) % M)) % M; // bC(p-i)
        ll temp = (firstTeam*secondTeam) % M;
        ans = (ans+temp) % M;
    }
 
    cout << ans;
}

This is a Bisection problem. For time $$$t$$$ seconds, $$$i^{th}$$$ student can be anywhere within segment $$$[x_{i}-v_{i}t , x_{i}+v_{i}t]$$$. In the binary search we need to check within $$$t$$$ seconds whether there is a common point among all the segments.

For all segments $$$[l_{1}, r_{1}] ,..., [l_{n}, r_{n}]$$$, we can calculate min $$$r_{i}$$$ and max $$$l_{i}$$$.

If $$$r_{i} < l_{i}$$$, there is no intersection. Otherwise [max $$$l_{i}$$$, min $$$r_{i}$$$ ] is the intersection.

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

// Function to check if all students can meet at a common point within time t
bool canMeet(double t, const vector<int>& positions, const vector<int>& speeds) {
    double min_meet = -1e9;
    double max_meet = 1e9;
    
    int n = positions.size();
    for (int i = 0; i < n; ++i) {
        double left_bound = positions[i] - speeds[i] * t;
        double right_bound = positions[i] + speeds[i] * t;
        min_meet = max(min_meet, left_bound);
        max_meet = min(max_meet, right_bound);
        
        if (min_meet > max_meet) {
            return false;
        }
    }
    return true;
}

int main() {
    ios_base::sync_with_stdio(0); cin.tie(0);
    int n;
    cin >> n;
    
    vector<int> positions(n);
    vector<int> speeds(n);
    
    for (int i = 0; i < n; ++i) {
        cin >> positions[i];
    }
    for (int i = 0; i < n; ++i) {
        cin >> speeds[i];
    }
    
    double left = 0;
    double right = 1e9;
    double precision = 1e-7;
    
    while (right - left > precision) {
        double mid = left + (right - left) / 2;
        if (canMeet(mid, positions, speeds)) {
            right = mid;
        } else {
            left = mid;
        }
    }
    
    cout << fixed << setprecision(10) << left;
}

...

#include<bits/stdc++.h>
using namespace std;           
 
int main(){
    ios_base::sync_with_stdio(false);cin.tie(NULL);
    int n;
    cin>>n;
    string s;
    cin>>s;
    int mx = 2;
    for(int i=0; i<s.size(); i++){
    	if(s[i]>='0' and s[i]<='9')mx = max(mx,s[i]-'0'+1);
    	else mx = max(mx, s[i]-'A'+11);
    }
    cout<<mx<<endl;
}

...

#include<bits/stdc++.h>
using namespace std;
int main(){
    int n;
    cin>>n;
    vector<int>chocolate(n);
    int total=0;
    for(int i=0;i<n;i++){
        cin>>chocolate[i];
    }
    for(int i=0;i<n;i++)total+=chocolate[i];
    vector<int>allpossiblesum(total+1,0);
    allpossiblesum[0]=1;
    for(int i=0;i<n;i++){
        for(int j=total;j>=chocolate[i];j--){
            allpossiblesum[j]|=allpossiblesum[j-chocolate[i]];
        }
    }
    int ans=total;
    for(int i=0;i<=total;i++){
        if(allpossiblesum[i]>0){
            int alice=i;
            int bob=total-alice;
            ans=min(ans,abs(alice-bob));
        }
    }
    cout<<ans<<endl;
}

If a player moves some peanuts from box $$$i$$$ to $$$i+1$$$, in the next move the other player can also move these peanuts from box $$$i+1$$$ to $$$i$$$. So first operation has no effect in this game. Suppose a player moves some peanuts from box $$$2$$$ to $$$1$$$. Then the other player can move these peanuts from box $$$1$$$ to $$$0$$$.

For all $$$i^{th}$$$ box where $$$i$$$ is even, if a player moves peanuts from it, the opposite player will make the last move. So these moves also have no effect. For determining the winner, we only need to xor all peanuts from $$$i^{th}$$$ box where $$$i$$$ is odd.

If you are wondering why we are doing xor, you should learn Nim strategy.

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

int main()
{
    ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
    int t; cin >> t;
    while (t--) {
        int n, k;
        cin >> n >> k;
        vector<ll> ar(n);
        for (int i = 0; i < n; i++) cin >> ar[i];

        ll xor_sum = 0;
        for (int i = 1; i < n; i += 2) xor_sum ^= ar[i];

        if (xor_sum == 0) cout << "NO" << endl;
        else cout << "YES" << endl;
    }
}

...

#include<bits/stdc++.h>
using namespace std;
 
int main(){
    ios_base::sync_with_stdio(false);cin.tie(NULL);
    int n;
    cin>>n;
    string s, p;
    cin >> s >> p;
    s="#"+s;
    p="#"+p;
    
    int pre_s[n+2][26];
    int pre_p[n+2][26];
    mem(pre_s, 0);
    mem(pre_p, 0);
    
    for(int i=1; i<=n; i++){
    	pre_s[i][s[i]-'a']=1;
    	pre_p[i][p[i]-'a']=1;
    	
    	for(int j=0; j<26; j++){
    		pre_s[i][j] += pre_s[i-1][j];
    		pre_p[i][j] += pre_p[i-1][j];
    	}
    }
    
    int q;
    cin>>q;
    while(q--){
    	int l, r;
    	cin>>l>>r;
    	int ans = 0;
    	for(int i=0; i<26; i++){
    		ans += abs((pre_s[r][i] - pre_s[l-1][i]) - (pre_p[r][i] - pre_p[l-1][i]));
    	}
    	cout<<ans<<endl;
    }
}

...

#include<bits/stdc++.h>
using namespace std;         
#define mxn 2000000
ll chk[mxn+5];

void sieve(){
    for(int i=2; i<mxn; i++){
	if(chk[i]==0){
	    for(int j=i+i; j<mxn; j+=i){
		chk[j] = 1;
	    }
	}
    }
}

int main(){
    ios_base::sync_with_stdio(false);cin.tie(NULL);
    sieve();
    ll n;
    cin>>n;
    ll ans = 1000000000000000ll;
    for(ll i=2; i<mxn; i++){
    	if(chk[i]) continue;
    	ll x = i*i;
    	ans = min(ans,abs(x-n));
    }

    cout<<ans<<endl;
}

...

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
 
signed main()
{
    ios_base::sync_with_stdio(0); cin.tie(0);
    ll n; cin >> n;
    if (n == 0 or n == 2) cout << "Same";
    else if (n == 1) cout << "Double";
    else cout << "Square";
}

...

#include<bits/stdc++.h>
using namespace std;
#define AS 250000
int i,j,k,l,m,n,p,q,r,a,b,c,u,v,x,y,z,ts=1;
int ar[AS],discover[AS],vis[AS],low[AS],br[AS];
vector<int> graph[AS];
set<int>st[AS];
vector<pair<int,int>>bridge;
void dfs(int nod,int par=-1) {
    discover[nod]=low[nod]=z++;
    vis[nod]=1;
    for(auto i:graph[nod]) {
        if(i==par)continue;
        if(vis[i])low[nod]=min(low[nod],discover[i]);
        else {
            dfs(i,nod);
            low[nod]=min(low[nod],low[i]);
 
            if(low[i]>discover[nod]) {
                bridge.push_back({nod,i});
                st[nod].erase(i);
                st[i].erase(nod);
            }
        }
    }
}
 
int component_count;
void biparted(int nod,int col)
{
    component_count++;
    vis[nod]=2;
    ar[nod]=col;
    for(auto i:st[nod]) {
        if(vis[i]==1)biparted(i,!col);
        else if(ar[nod]==ar[i])c=1;
    }
}
int main() {
    cin>>n>>m;
    for(int i=1;i<=m;i++) {
        cin>>u>>v;
        graph[u].push_back(v);
        graph[v].push_back(u);
        st[u].insert(v);
        st[v].insert(u);
    }
    c=0;
    for(int i=1;i<=n;i++) {
        if(vis[i]==0) {
            dfs(i,-1);
        }
    }
    c=0;
    int ans=0;
    for(int i=1;i<=n;i++) {
        if(vis[i]==1) {
            component_count=c=0;
            biparted(i,0);
            if(c)ans+=component_count;
        }
    }
    cout<<ans<<endl;
}