比較函式return true 意味著排序需要交換。

#include <iostream>
#include <queue>
#include <vector>
#include <algorithm>

using namespace std;

struct Item {
    int val = 0;
    int idx = 0;
};

/**
 * want ascending order, if ties, prefer less idx
 * return true if we want swap happen
 *
 
 
*/
class Compare {
public:
    bool operator() (const Item &lhs, const Item &rhs) {
        if (lhs.val == rhs.val) {
            return lhs.idx > rhs.idx;
        }
        return lhs.val < rhs.val;
    }
};

int main() {
    Item item1;
    item1.val = 1;
    item1.idx = 1;
    Item item2;
    item2.val 
 
= 2;
    item2.idx = 2;
    vector<Item> v = {item1, item2};
    priority_queue<Item, vector<Item>, Compare> myPQ(v.begin(),v.end());
    cout << myPQ.top().idx << endl;
    std::sort(v.begin(),v.end(), Compare());
    cout << v[0].val << endl;
    return 0;
}