题目如下：

The following iterative sequence is defined for the set of positive integers:

n → n/2 (n is even)
n → 3n + 1 (n is odd)

Using the rule above and starting with 13, we generate the following sequence:

13 → 40 → 20 → 10 → 5 → 16 → 8 → 4 → 2 → 1

It can be seen that this sequence (starting at 13 and finishing at 1) contains 10 terms. Although it has not been proved yet (Collatz Problem), it is thought that all starting numbers finish at 1.

Which starting number, under one million, produces the longest chain?

NOTE: Once the chain starts the terms are allowed to go above one million.

此题可以直接做，但是可能效率低，需要借助一点动态规划思路，减少运算次数

Matlab代码如下

n = 1e6;
c = zeros(1e6,1);
c(1) = 1;
for i = 2 : n[t,s] = deal(0,i);while s >= iif mod(s,2) == 0s = s/2;elses = s*3+1;endt = t + 1;endc(i) = t + c(s);
end
[~,idx] = max(c)

PS: if语句可以优化掉