tape and a tray where letters are stacked one on top of another. The tray starts empty. While there are letters on the tape or on the tray, you may do either of two things:
- take the **first** letter off the tape and put it on top of the tray;
- take the **top** letter off the tray and print it on paper (it is appended to what has been printed so far).
In the end every letter ends up on paper. Return the lexicographically smallest string that can be printed this way.tape='dcab' → 'abcd'
Put d, c and a on the tray and print a. The top of the tray is now c, but b is still on the tape and smaller, so take b and print it, then print c and d. The result is "abcd".
tape='cbc' → 'bcc'
Put c and b on the tray and print b. The top is c and the tape holds only another c, nothing smaller, so print c, then take and print the last c: "bcc".
tape='ba' → 'ab'
tape.length ≤ 10⁵tape consists of lowercase Latin letters onlytape='dcab'
'abcd'
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