为什么下面这个代码是78分

#include<bits/stdc++.h>
using namespace std;
struct node
{
	int qd, zd, qz;
}a[50005];
int b[1505], n, m;
int main()
{
	memset(b, -0x3f, sizeof(b));
	cin >> n >> m;
	for(int i = 1; i <= m; i++)
	{
		cin >> a[i].qd >> a[i].zd >> a[i].qz;
		//a[i].qz = -a[i].qz;
	}
	b[1] = 0;
	for(int i = 1; i <= n - 1; i++)
	{
		for(int j = 1; j <= m; j++)
		{
			b[a[j].zd] = max(b[a[j].qd] + a[j].qz, b[a[j].zd]);
		}
	}
	if(b[n] == -0x3f3f3f3f) cout << -1 << endl;
	else cout << b[n] << endl;
	return 0;
}

但是这个代码就A了？

#include<bits/stdc++.h>
using namespace std;
struct node
{
	int qd, zd, qz;
}a[50005];
int b[1505], n, m;
int main()
{
	memset(b, 0x3f, sizeof(b));
	cin >> n >> m;
	for(int i = 1; i <= m; i++)
	{
		cin >> a[i].qd >> a[i].zd >> a[i].qz;
		a[i].qz = -a[i].qz;
	}
	b[1] = 0;
	for(int i = 1; i <= n - 1; i++)
	{
		for(int j = 1; j <= m; j++)
		{
			b[a[j].zd] = min(b[a[j].qd] + a[j].qz, b[a[j].zd]);
		}
	}
	if(b[n] == 0x3f3f3f3f) cout << -1 << endl;
	else cout << -b[n] << endl;
	return 0;
}

求大佬解惑！！！