View Code of Problem 128

#include<iostream>
#include<vector>
#include<algorithm>
#include<string>
#include<climits>
#include<cmath>
#include<map>
#include<set>

using namespace std;


vector<vector<int>> city(1001, vector<int>(1001, 0));

//bool dfs(int time, int pos, int n) {
//
//	if (pos == 1 && time < 0)
//		return true;
//	if (pos == 1 && time >= 0)
//		return false;
//
//	for (int i = n - 1; i >= 1; i--) {
//
//		if (city[i][pos] != 0) {	//存在路径
//
//			if (dfs(time + city[i][pos], i, n))
//				return true;
//		}
//	}
//
//	return false;
//}


int main()
{
	int m, n;

	while (cin >> n >> m) {

		vector<vector<int>> dp(n + 1, vector<int>(n + 1, INT_MAX));

		for (int i = 0; i < m; i++) {

			int a, b, t;
			cin >> a >> b >> t;

			city[a][b] = t;
		}

		for (int i = 2; i <= n; i++) {

			if (city[1][i] != 0)
				dp[1][i] = city[1][i];	//赋初值
		}


		for (int i = 2; i <= n - 1; i++) {

			for (int j = 2; j <= n; j++) {

				if (city[i][j] != 0) {

					dp[1][j] = min(dp[1][j], dp[1][i] + city[i][j]);
				}
			}

			if (dp[1][n] < 0)
				break;
		}

		
		if(dp[1][n] < 0)
			cout << "YES" << endl;
		else
			cout << "NO" << endl;

		//if (dfs(0, n, n))
		//	cout << "YES" << endl;
		//else
		//	cout << "NO" << endl;
	}
}