P1073 最优贸易

#include <bits/stdc++.h>
using namespace std;
const int maxn = 100005;
int head1[maxn], head2[maxn], maxx[maxn], minn[maxn];
bool in1[maxn], in2[maxn];
int value[maxn];
int N, m, s;
struct edge
{
	int to, next;
}es1[maxn], es2[maxn];
queue<int> q;
int num1 = 0, num2 = 0;
void link(int x, int y) {
	num1++;
	es1[num1].to = y;
	es1[num1].next = head1[x];
	head1[x] = num1;
	num2++;
	es2[num2].to = x;
	es2[num2].next = head2[x];
	head2[y] = num2;
}
void spfa1() {
	memset(minn, 127, sizeof(minn));
	minn[s] = value[s];
	in1[s] = 1;
	q.push(s);
	while(!q.empty()) {
		int x = q.front();
		q.pop();
		in1[x] =false;
		for(int i = head1[x]; i != -1; i = es1[i].next) {
			int n = es1[i].to;
			if(minn[n] > minn[x] || value[n] < minn[n]) {
				minn[n] = min(minn[x], value[n]);
				if(in1[n] == 0) {
					q.push(n);
					in1[n] = true;
				}
			}
		}
	}
}
void spfa2() {
	memset(maxx, -1, sizeof(maxx));
	maxx[N] = value[N];
	in2[N] = 1;
	q.push(N);
	while(!q.empty()) {
		int x = q.front();
		q.pop();
		in2[x] =false;
		for(int i = head2[x]; i != -1; i = es2[i].next) {
			int n = es2[i].to;
			if(maxx[n] < maxx[x] || value[n] > maxx[n]) {
				maxx[n] = max(maxx[x], value[n]);
				if(in2[n] == 0) {
					q.push(n);
					in2[n] = true;
				}
			}
		}
	}
}
int main() {
//	freopen("input.in", "r", stdin);
	memset(head1, -1, sizeof(head1));
	memset(head2, -1, sizeof(head2));
	memset(in1, 0, sizeof(in1));memset(in2, 0, sizeof(in2));
	cin >> N >> m;
	s = 1;
	for(int i = 1; i <= N; i++) cin >> value[i];
	for(int i = 1; i <= m; i++) {
		int x, y, z;
		cin >> x >> y >> z;
		link(x, y);
		if(z == 2) link(y, x);
	}
	spfa1();
	spfa2();
	int ans = 0;
	for(int i = 1; i <= N; i++) {
		ans = max(maxx[i]-minn[i], ans);
	}
	cout << ans;
}

明白了一个道理：spfa不仅仅能求最短路啊！

以后要记住，如果要求求一条路上的某个值，可以考虑用spfa