This program illustrates two constructors of the STL priority-queue class,
and the assignment of one priority_queue object to another.
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First, we create an empty priority_queue object pq1, then push five values
into it, then display those five values in the default priority order.
Popping 46
Popping 35
Popping 27
Popping 19
Popping 11
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Second, we create a priority_queue pq2 with four values from a list,
then display those four values, again in default priority order.
Popping 44
Popping 33
Popping 22
Popping 11
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Finally, we display pq3, which was obtained by assigning pq2 to pq3,
immediately after declaring pq3 as an empty priority_queue object.
Popping 44
Popping 33
Popping 22
Popping 11
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