Ruudukko

#include <iostream>
#include <set>
#define F first
#define S second
using namespace std;

int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  int n;
  cin >> n;
  string t[n];
  for (int i = 0; i < n; ++i) cin >> t[i];
  string r;
  set<pair<int, int>> w;
  w.insert({0, 0});
  r.push_back(t[0][0]);
  for (int i = 0; i < 2*n-2; ++i) {
    set<pair<int, int>> nw;
    char mn = 'Z';
    for (auto p : w) {
      if (p.F < n-1) {
        char ov = t[p.F+1][p.S];
        if (ov <= mn) {
          if (ov < mn) {
            nw.clear();
            mn = ov;
          }
          nw.insert({p.F+1, p.S});
        }
      }
      if (p.S < n-1) {
        char ov = t[p.F][p.S+1];
        if (ov <= mn) {
          if (ov < mn) {
            nw.clear();
            mn = ov;
          }
          nw.insert({p.F, p.S+1});
        }
      }
    }
    r.push_back(mn);
    w = std::move(nw);
  }
  cout << r << endl;
}

Test 1

Group: 1

Verdict: ACCEPTED

Test 2

Group: 1

Verdict: ACCEPTED

Test 3

Group: 1

Verdict: ACCEPTED

Test 4

Group: 1

Verdict: ACCEPTED

Test 5

Group: 1

Verdict: ACCEPTED

Test 6

Group: 2

Verdict: ACCEPTED

Test 7

Group: 2

Verdict: ACCEPTED

Test 8

Group: 2

Verdict: ACCEPTED

Test 9

Group: 2

Verdict: ACCEPTED

Test 10

Group: 2

Verdict: ACCEPTED

Test 11

Group: 3

Verdict: ACCEPTED

Test 12

Group: 3

Verdict: ACCEPTED

Test 13

Group: 3

Verdict: ACCEPTED

Test 14

Group: 3

Verdict: ACCEPTED

Test 15

Group: 3

Verdict: ACCEPTED