使用STL优先队列的最小堆说明

Question

I am fairly new to C++ and I was trying to implement a Min Heap using Priority Queue STL in C++. I looked around the web and chanced upon a code, the link to which I have added at the bottom.

I understand how vectors work and I also understand what a functor is but I can't understand how 'return i>j' plays a part in arranging the elements from minimum to maximum.

I will post the code here.

#include <queue>
#include <iostream>

using namespace std;

struct comparator {
     bool operator()(int i, int j) {
     return i > j;
    }
};

int main(int argc, char const *argv[])
{
     priority_queue<int, std::vector<int>, comparator> minHeap;

    minHeap.push(10);
    minHeap.push(5);
    minHeap.push(12);
    minHeap.push(3);
    minHeap.push(3);
    minHeap.push(4);

    while (!minHeap.empty()) {
    cout << minHeap.top() << " ";
    minHeap.pop();
    }
    return 0;
}

Could someone please trace the program's execution? I have been trying to understand how this program works for the past couple of hours with no luck.

I got the code from here

Answer 1

Priority queues first element is always the greatest of the elements it contains, according to a given criterion. This criterion is your functor comparator . In this case, it's defining that for a two given elements the minor one should be first.

Hence, in your code, if we extract all the elements the output will be the element ordered min to max:

while (!minHeap.empty()) {
   cout << minHeap.top() << " ";
   minHeap.pop();
}
//output: 3,3,4,5,10,12

The algorithm to push an element is like a heap:

1. Place the new element in the next available position in the array. (lower level of the tree)
2. Compare the new element with its parent using the functor provided ( comparator ). In this case, you comparison is "if the new element is smaller", then swap it with its parent.
3. Continue this process until either:
   • the new element's parent is smaller than or equal to the new element, or
   • the new element reaches the root (index 0 of the array)

Maybe it is more clear to see how the algorithm works with a visual example: Imagine you have already pushed the numbers 6,3,5,9 and now you push 2.