题面

传送门

Candies

Time Limit: 1500MS Memory Limit: 131072K
Total Submissions: 48558 Accepted: 13814

Description

During the kindergarten days, flymouse was the monitor of his class. Occasionally the head-teacher brought the kids of flymouse’s class a large bag of candies and had flymouse distribute them. All the kids loved candies very much and often compared the numbers of candies they got with others. A kid A could had the idea that though it might be the case that another kid B was better than him in some aspect and therefore had a reason for deserving more candies than he did, he should never get a certain number of candies fewer than B did no matter how many candies he actually got, otherwise he would feel dissatisfied and go to the head-teacher to complain about flymouse’s biased distribution.

snoopy shared class with flymouse at that time. flymouse always compared the number of his candies with that of snoopy’s. He wanted to make the difference between the numbers as large as possible while keeping every kid satisfied. Now he had just got another bag of candies from the head-teacher, what was the largest difference he could make out of it?

Input

The input contains a single test cases. The test cases starts with a line with two integers N and M not exceeding 30 000 and 150 000 respectively. N is the number of kids in the class and the kids were numbered 1 through N. snoopy and flymouse were always numbered 1 and N. Then follow M lines each holding three integers A, B and c in order, meaning that kid A believed that kid B should never get over c candies more than he did.

Output

Output one line with only the largest difference desired. The difference is guaranteed to be finite.

Sample Input

2 2
1 2 5
2 1 4

Sample Output

5

Hint

32-bit signed integer type is capable of doing all arithmetic.

题目大意

发糖，为了让所有人开心起见，存在若干对A与B的攀比关系，A的糖数不能比B的少于某个值。最终需要找到1和n的差值的最大值。

解题思路

没什么好说的，就是一个最短路题1->n的最短路。

踩坑

平时写dij用惯了vector了，这题数据较大，劣势就显现出来了。vector会开辟一定的初始化空间，空间不够用时会重新申请内存（新内存大小由编译器决定，MSVC是1.5倍策略，G++是2倍策略），然后将原数据拷贝过去，这个过程很耗时！当数据量较大时就会劣势尽显，反复拷贝会浪费大量的时间，容易导致程序TLE。
所以建议存边用链表，从而避免卡时间。