static class Grid {
  int[] mem = new int[0];
  
  *() {}
  *(int[] data) { set(0, data); }
  
  void set(int addr, int x) {
    if (addr < 0) ret;
    expandTo(addr);
    mem[addr] = x;
  }
  
  void set(int addr, int[] x) {
    for (int i = l(x)-1; i >= 0; i--)
      set(addr+i, x[i]);
  }
  
  int get(int addr) {
    if (addr < 0 || addr >= l(mem))
      ret 0;
    ret mem[addr];
  }

  void expandTo(int n) {
    if (n >= l(mem)) {
      int[] m = new int[max(n+1, l(mem)*2)];
      arraycopy(mem, 0, m, 0, l(mem));
      mem = m;
    }
  }
  
  int lastNonZero() {
    int i = l(mem)-1;
    while (i > 0 && get(i) == 0) --i;
    ret i;
  }
  
  int[] getRange(int a, int b) {
    ret subArray(mem, a, b);
  }
  
  void swap(int a, int b) {
    int i = get(a);
    set(a, get(b));
    set(b, i);
  }
}

Began life as a copy of #1003139