A string has been compressed with one rule: k[text] stands for text written out k times in a row. Groups can sit inside other groups. Expand s completely and return the result.
Letters outside any brackets are copied as they are. Digits only ever appear as a repeat count directly before a [, so there is no input like 4b or 3[7].
Expand from the inside out: 3[y] is yyy, so the group becomes 2[xyyy], which is xyyyxyyy. The trailing z follows once.
A count can have more than one digit: 10[o] is ten os.
Constraints
1 <= s.length <= 30s holds lowercase English letters, digits and the brackets [ ].s is a valid encoding: brackets are balanced, every [ comes right after its count, and no pair of brackets is empty.1 to 300, written without leading zeros.10⁵ characters long.A group can't be expanded until you reach its closing ], and the innermost group closes first: that last-opened, first-closed order is what a stack handles.