gistfile1.txt

           col=1 col=2 col=3 col=4
row=1 0     1       2         3       4
row=2 5     6       7         8       9
...




1,2,,,,n


For example, here is a log file.
First/Second column is member id.
Third column is a timestamp

1 2 201903161010
2 3 201903161011
...

code is something like this.
If we use a weithted quick-union, running time of union method is logn
checkAllMembersConnected is also logn but unfortunately I can't explain clearly...
Anyway all running time is at most mlogn

for(int i=0; i<m; i++) {
    union(firstMembers[i], secondMembers[i]);
    if(checkAllMembersConnected()) {
        return timestamp[i];
        break;
    }
}


private int[] firstMembers;
private int[] secondMembers;
private int[] timestamp;

for(int i=0; i<m; i++) {
	union(firstMembers[i], secondMembers[i]);
	if(checkAllMembersConnected()) {
	  return timestamp[i];
	  break;	
	}
}

public boolean checkAllMembersConnected() {
boolean allMembersConnectedFlag=true
for(int i=0; i<n-1; i++) {
	if(!connected(i, i+1)) {
		allMembersConnectedFlag=false;
		break;
	}
}