Build output from popped pieces
Updated
The final output is the fragment left after the last ]. Pieces come from pops, not from a second pass. Printing the stack of fragments at the end (bottom to top) is not the decode. Those are unfinished checkpoints.
What to focus on
After a well-formed decode, the checkpoint stack is empty and the current fragment is the answer. Leftover letters after a group append to that fragment.
Worked examples
done
s = "3[ab]" after last ]- stack of checkpoints empty
- frag ababab
Result: "ababab"
unclosed
s = "3[ab"- checkpoint still on the stack
Result: invalid / incomplete
concat leftover letters
s = "3[ab]c"- after ] frag ababab then append c
Result: "abababc"
Try it
Write decode(s). s is a well-formed decode string. Return the current fragment after the last ]. The checkpoint stack is empty at the end. Leftover letters append to that fragment. s = \"3[ab]\" after last ] → \"ababab\". s = \"3[ab\" → invalid / incomplete. s = \"3[ab]c\" → \"abababc\".
s is a well-formed decode string. Matching brackets, positive counts. The checkpoint stack is empty at the end.