com certeza é melhor xD

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

const int N = 1006;
int t, n, s[N], e[N], o[N], ans[N];

int main() {
  scanf("%d", &t);
  for (int tt = 1; tt <= t; tt++) {
    printf("Case #%d: ", tt);

    scanf("%d", &n);
    for (int i = 0; i < n; i++) scanf("%d%d", &s[i], &e[i]), o[i] = i;
    sort(o, o+n, [](int a, int b){ return s[a] == s[b] ? e[a] < e[b] : s[a] < s[b]; });

    int ok = 1;
    int c = 0, j = 0;
    for (int i = 0; i < n and ok; i++) {
      int x = o[i];

      /**/ if (c <= s[x]) ans[x] = 'C', c = e[x];
      else if (j <= s[x]) ans[x] = 'J', j = e[x];
      else ok = 0;
    }

    if (!ok) {
      printf("IMPOSSIBLE\n");
    } else {
      for (int i = 0; i < n; i++) putchar(ans[i]);
      printf("\n");
    }
  }

  return 0;
}