stack1
and stack2
.stack1
if it is not null.stack1
is not empty:
a. Pop a node from stack1
and push it onto stack2
.
b. Push the left child of the popped node onto stack1
if it exists.
c. Push the right child of the popped node onto stack1
if it exists.stack2
will contain the nodes in postorder sequence. Pop all nodes from stack2
and add them to the output list.