Shared code

#include<bits/stdc++.h>
using namespace std;
void __print(int x) {cerr << x;}
void __print(long x) {cerr << x;}
void __print(long long x) {cerr << x;}
void __print(unsigned x) {cerr << x;}
void __print(unsigned long x) {cerr << x;}
void __print(unsigned long long x) {cerr << x;}
void __print(float x) {cerr << x;}
void __print(double x) {cerr << x;}
void __print(long double x) {cerr << x;}
void __print(char x) {cerr << '\'' << x << '\'';}
void __print(const char *x) {cerr << '\"' << x << '\"';}
void __print(const string &x) {cerr << '\"' << x << '\"';}
void __print(bool x) {cerr << (x ? "true" : "false");}
 
template<typename T, typename V>
void __print(const pair<T, V> &x);
template<typename T>
void __print(const T &x) {int f = 0; cerr << '{'; for (auto &i: x) cerr << (f++ ? ", " : ""), __print(i); cerr << "}";}
template<typename T, typename V>
void __print(const pair<T, V> &x) {cerr << '{'; __print(x.first); cerr << ", "; __print(x.second); cerr << '}';}
void _print() {cerr << "]\n";}
template <typename T, typename... V>
void _print(T t, V... v) {__print(t); if (sizeof...(v)) cerr << ", "; _print(v...);}
//#ifdef DEBUG
#define dbg(x...) cerr << "\e[91m"<<__func__<<":"<<__LINE__<<" [" << #x << "] = ["; _print(x); cerr << "\e[39m" << endl;
//#else
//#define dbg(x...)
//#endif
 
typedef long long ll;
typedef long double ld;
typedef complex<ld> cd;
 
typedef pair<int, int> pi;
typedef pair<ll,ll> pl;
typedef pair<ld,ld> pd;
 
typedef vector<int> vi;
typedef vector<ld> vd;
typedef vector<ll> vl;
typedef vector<pi> vpi;
typedef vector<pl> vpl;
typedef vector<cd> vcd;
 
template<class T> using pq = priority_queue<T>;
template<class T> using pqg = priority_queue<T, vector<T>, greater<T>>;
 
#define rep(i, a) for(int i=0;i<a;++i)
#define FOR(i, a, b) for (int i=a; i<(b); i++)
#define F0R(i, a) for (int i=0; i<(a); i++)
#define FORd(i,a,b) for (int i = (b)-1; i >= a; i--)
#define F0Rd(i,a) for (int i = (a)-1; i >= 0; i--)
#define trav(a,x) for (auto& a : x)
#define uid(a, b) uniform_int_distribution<long long>(a, b)(rng)
 
#define sz(x) (int)(x).size()
#define mp make_pair
#define pb push_back
//#define f first
//#define s second
#define lb lower_bound
#define ub upper_bound
#define all(x) x.begin(), x.end()
#define ins insert
 
template<class T> bool ckmin(T& a, const T& b) { return b < a ? a = b, 1 : 0; }
template<class T> bool ckmax(T& a, const T& b) { return a < b ? a = b, 1 : 0; }
 
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
 
const char nl = '\n';
const int N = 3001;
const int INF = 1e9+7;
const long long LINF = 1e18+7;
 
bool vst[N][N];
 
int main(){
    ios::sync_with_stdio(false);cin.tie(nullptr);
    int n; cin >> n;
    vector<string> g(n+1);
    for(int i=1;i<=n;++i){
        cin >> g[i];
        g[i] = 'x' + g[i];
    }
    
    string ans = ""+g[1][1];
    queue<pair<int, int>> q1, q2;
    q1.push({1, 1});
    vst[1][1] = 1;
    ans.pb(g[1][1]);
    bool ok = 0;
    while(!ok){
        char gmin = 'Z';
        vector<pair<int, int>> temp;
        while(!q1.empty()){
            auto [i, j] = q1.front(); q1.pop();
            if(i == n and j == n)ok = 1;
            if(i + 1 <= n){
                if(!vst[i+1][j]){
                    vst[i+1][j] = 1;
                    temp.pb({i+1, j});
                    if(g[i+1][j] < gmin)gmin = g[i+1][j];
                }
            }
 
            if(j + 1 <= n){
                if(!vst[i][j+1]){
                    vst[i][j+1] = 1;
                    temp.pb({i, j+1});
                    if(g[i][j+1] < gmin)gmin = g[i][j+1];
                }
            }
        }
        
        if(ok)break;
        ans.pb(gmin);
 
        for(auto &p:temp){
            if(g[p.first][p.second] == gmin)q2.push(p);
        }
 
        swap(q1, q2);
    }
 
    cout << ans << nl;
}