为什么这样写会错

int dijkstra()//错误
{
    memset(dist, 0x3f, sizeof(dist));
    dist[1] = 0;
    priority_queue<PII, vector<PII>, greater<PII>> heap;
    heap.push({ 0, 1 });
    st[1]=true;
    while(heap.size())
    {
        PII k = heap.top(); 
        heap.pop();
        int ver = k.second, distance = k.first;

        for(int i = h[ver]; i != -1; i = ne[i])
        {
            int j = e[i]; 
            if(dist[j] > distance + w[i])
            {
                dist[j] = distance + w[i];
                if(!st[j])heap.push({ dist[j], j }),st[j]=true;//在入堆前判重
            }
        }
    }
    if(dist[n] == 0x3f3f3f3f) return -1;
    else return dist[n];
}

正解：

#include<iostream>
#include<cstring>
#include<queue>

using namespace std;

typedef pair<int, int> PII;

const int N = 100010; 


int h[N], e[N], ne[N], idx;
int w[N]; 
int dist[N];
bool st[N]; 

int n, m;

void add(int x, int y, int c)
{
    w[idx] = c;
    e[idx] = y;
    ne[idx] = h[x]; 
    h[x] = idx++;
}

int dijkstra()
{
    memset(dist, 0x3f, sizeof(dist));
    dist[1] = 0;
    priority_queue<PII, vector<PII>, greater<PII>> heap;
    heap.push({ 0, 1 });
    while(heap.size())
    {
        PII k = heap.top(); 
        heap.pop();
        int ver = k.second, distance = k.first;

        if(st[ver]) continue;
        st[ver] = true;

        for(int i = h[ver]; i != -1; i = ne[i])
        {
            int j = e[i]; 
            if(dist[j] > distance + w[i])
            {
                dist[j] = distance + w[i];
                heap.push({ dist[j], j });
            }
        }
    }
    if(dist[n] == 0x3f3f3f3f) return -1;
    else return dist[n];
}

int main()
{
    memset(h, -1, sizeof(h));
    scanf("%d%d", &n, &m);

    while (m--)
    {
        int x, y, c;
        scanf("%d%d%d", &x, &y, &c);
        add(x, y, c);
    }

    cout << dijkstra() << endl;

    return 0;
}
/*
此代码作者：optimjie//我不是作者
链接：https://www.acwing.com/solution/content/6554/
来源：AcWing
著作权归作者所有。
  */