Character.compare('1','2'); //out: -1
This output means that that "2:..." will go before "1:..." as required in the solution;

If we have "1:bbb" and "2:aaa" then result will be "1:bbb/2:aaa", when the right answer is "2:aaa/1:bbb"

It is compared from the beginning, including the 1,2 and =,
if it is the same,then ":",same,then the letter lexicographic order

if (m.containsKey(e)) m.put(e, m.get(e) + 1);
else m.put(e, 1);

map.merge(e, 1, (a, b) -> a + b);