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  • P4448Joint Stacks
    Limits : Time Limit : - MS   Memory Limit : - KB
    Judgment Tips : 时限1s,空限128m
    Description

    A stack is a data structure in which all insertions and deletions of entries are made at one end, called the "top" of the stack. The last entry which is inserted is the first one that will be removed. In another word, the operations perform in a Last-In-First-Out (LIFO) manner.
    A mergeable stack is a stack with "merge" operation. There are three kinds of operation as follows:

    - push A x: insert x into stack A
    - pop A: remove the top element of stack A
    - merge A B: merge stack A and B

    After an operation "merge A B", stack A will obtain all elements that A and B contained before, and B will become empty. The elements in the new stack are rearranged according to the time when they were pushed, just like repeating their "push" operations in one stack. See the sample input/output for further explanation.
    Given two mergeable stacks A and B, implement operations mentioned above.

    Input Format

     the first line contains an integer N(0<N≤10^5), indicating the number of operations.
    The next N lines, each contain an instruction "push", "pop" or "merge".
    The elements of stacks are 32-bit integers. Both A and B are empty initially, and it is guaranteed that "pop" operation would not be performed to an empty stack.
     

    Output Format

    For each "pop" operation, output the element that is popped, in a single line.

    Sample Input 1

    4
    push A 1
    push A 2
    pop A
    pop A

    Sample Output 1

    2
    1

    Sample Input 2

    9
    push A 0
    push A 1
    push B 3
    pop A
    push A 2
    merge A B
    pop A
    pop A
    pop A

    Sample Output 2

    1
    2
    3
    0

    Sample Input 3

    9
    push A 0
    push A 1
    push B 3
    pop A
    push A 2
    merge B A
    pop B
    pop B
    pop B 

    Sample Output 3

    1
    2
    3
    0


    Source  hdu 5818