187.

After entering the 2900th question, Cheng Li found that the next 90 questions are all closely related to the field of electronic computers.

Question 2900: "How do I use a machine that mechanically calculates automatically?"

Question 2901: "How do you build a machine with logic operations through logic switches?"

Question 2902: "Q, how to discriminate the mechanical solvability and computability of mathematical problems?"

......

Question 2910: "Ask, is there a discriminancy of the solvability of the Diophantine equation?" ”

......

These 90 questions cover math problems related to the field of computing.

For example, set theory and logic are crucial, as well as statistics, matrix theory, measure theory, differential manifolds, Li Qunlun, graph theory, chaotic dynamics, linear programming, ...... Wait a minute.

A lot of this is including the design of the algorithm.

For example, No. 2977 to the question: "Design an algorithm to calculate a problem: a salesman wants to go to several cities to sell goods, and the salesman starts from one city, needs to pass through all the cities, and then return to the starting place." Q, how should the route of travel be selected so that the total trip is the shortest. ”

This problem is the well-known traveling salesman's problem, which is an NP difficult problem in combinatorial optimization and is very important in operations research and theoretical computer science.

From the perspective of graph theory, the essence of this problem is to find a Hamiltonian loop with the smallest weight in a completely undirected graph with weight. Since the feasible solution to this problem is a full permutation of all vertices, a combinatorial explosion occurs as the number of vertices increases.

The above statement, in simple terms, is to list all the possible routes, calculate the total distance, and then compare them to find the shortest route.

The idea is simple, but it doesn't work when there are more than a certain number of cities.

Because the amount of computation is too large.

For example, when the number of cities reaches 20, it takes hundreds of years to calculate the shortest route among all possible routes in these 20 cities, even if a computer calculates hundreds of millions of times per second.

"Actually, this traveling salesman problem is similar to the algorithm I used to calculate the operation route map of "Heavenly Illusion Gong" in the meridians, and it belongs to the same nature. ”

Previously, when Cheng Li calculated the heaven-level exercises in the meridians, he had to calculate the route with the fewest number of vein rings from the 30,000 vein rings.

This problem, in fact, is more computationally intensive than the traveling salesman's problem.

Because the change of the pulse ring is equivalent to the distance between two cities in a traveling salesman. And there are many types of pulse ring changes, which are undoubtedly more complex than the distance between two cities.

Therefore, when calculating the heaven-level exercises, the algorithm designed by Cheng Li was only to limit a relatively optimized shortest possible route. And without going through these 30,000 rings, all the possible routes.

Otherwise, with the calculation speed of the B-type calculator, even if it is calculated for 10,000 years, it may not be possible to get an answer.

Later, when it came to specifically calculating the operation roadmap of "Heavenly Illusion Gong", it was also necessary to make the spiritual power flow appear 9 times in the process of operation.

There is no doubt that the complexity and amount of computation has gone up a new level.

Therefore, the algorithm designed by Cheng Li at that time also did not traverse all possible routes.

Instead, let the calculator output it directly every time it calculates a roadmap that meets the criteria.

Then slowly dig down to create a new route.

Therefore, every time a new route is calculated, the more computational time and computational effort it takes to dig out the new algorithm in the future, and it increases exponentially.

If you want that type B calculator to calculate 30,000 vein rings at once, all the eligible "Heavenly Illusion Gong" roadmaps that may exist may not be able to be calculated in hundreds of thousands of years.

Therefore, the algorithm designed by Cheng Li at that time did not use an exact algorithm, but a heuristic algorithm.

"The traveling salesman's problem, there are a lot of algorithms that can be calculated. Cheng Li pondered while answering the question on the light sand, "Early researchers used precise algorithms to calculate this problem, and the commonly used algorithms were: branch delimitation method, linear programming method, dynamic programming method, etc. But as the number of cities increases, precise algorithms will become powerless. Therefore, in later research, approximation algorithms and heuristic algorithms are basically used, such as: genetic algorithm, simulated annealing method, ant colony algorithm, taboo search algorithm, greedy algorithm and neural network......"

Cheng Li said as he quickly wrote down an algorithm design idea on the light sand.

"When I calculated the roadmap of the operation of "Heavenly Illusion Gong", I used a heuristic algorithm. Now let's design a heuristic to answer this question. ”

Cheng Li quickly wrote down an algorithm design idea in Guangsha.

And the moment he finished writing, the light point recombined the word "correct", and Cheng Li was able to move on to the next layer.

In this way, Cheng Li is moving forward rapidly in the 2901-2900 floors.

As a programmer who has studied mathematics, Cheng Li is no stranger to these things, and can even be said to be very familiar with them.

So in the first 90 questions of the final 100 sprints, Cheng Li answered quickly.

However, because most of the problems of the 2500-2900 layers are not familiar to Cheng Li and are extremely difficult, Cheng Li spent a lot of time.

It took 2000 hours to 2500 floors.

It took another 2500-21 hours on the 2900th floor.

By the time we got to the 2990th floor, it was 7 a.m. on June 14.

When Cheng Li passed through the 2900th floor and stepped into the 2901st floor, he looked at the time.

"It's already 7 o'clock in the morning on June 14th, and according to the previous estimates, the Blood Demon Island army should have just arrived on Qingling Island at this time...... There's still the last ten floors, and I've got to hurry!"

According to Cheng Li's estimate, if the last ten floors, like the previous 90 floors, are all computer-related problems, then it will not take him too long to pass, and he will soon be able to reach the 300th floor.

But when he saw the problem on the 2901st floor, he was a little confused.

"That's the problem? It's kind of bad now......"

......

Just when Cheng Li was in a difficult situation, at this time, Qingling Island was facing a bigger crisis!

After a day and a night of preparations, as well as the pre-war mobilization that Tuomu Zhenren had worked hard before, Qingling Island finally looked like a sharpening knife and preparing for a big war at this time.

However, when everyone saw that in the thick fog of the sky, endless blood light was suddenly reflected, and everyone couldn't help but feel nervous and fearful.

"The enemy is coming!

Accompanied by the deep voice of Takuki Zhenren resounded throughout the island.

In the sky above Qingling Island, thick blood light flashed out in all directions, and in the end, the blood light was so thick that it was about to drip blood from the mist.

Time seemed to freeze at this moment, and everyone seemed to feel that time had slowed down.

In the next second, an incomparably huge pool of blood, with a rich murderous aura, came out of the mist like this, standing in the sky above Qingling Island!

"Kill!"