Modify following code to implement two stacks sharing the same array, as shown inFigure $4\mathrm{.}21.$

$/**$ Array$-$based stack implementation

$*$ Constructors $*$ Reinitialize stack $*$ Push $"$it$"$ onto stack $*$ Remove and top element $*$ @return Top element $*$ @return Stack size $*/$

public int length$()$ return top;