I've fully solved a particular problem on HackerRank (https://www.hackerrank.com/challenges/ctci-recursive-staircase/problem) using a recursive solution with memoization:
std::map<int, int> memoize;
int davis_staircase(int n) {
if (n == 0) {
return 1;
} else if (n < 0) {
return 0;
}
auto find = memoize.find(n);
if (find != memoize.end()) {
return find->second;
}
int num_steps = davis_staircase(n - 1) + davis_staircase(n - 2) + davis_staircase(n - 3);
memoize[n] = num_steps;
return num_steps;
}
I would like to hide the global std::map
(without using a class) that I'm using as the lookup and thought I'd try creating a lambda that I can call recursively and also capture the cache/map by reference. I've tried the following:
int davis_staircase_2(int n) {
std::map<int, int> memo;
//auto recurse = [&memo](int n) -> int { // attempt (1)
//std::function<int(int)> recurse = [&memo](int n) -> int { // attempt (2)
std::function<int(std::map<int, int>&, int)> recurse = [](std::map<int, int>& memo, int n) -> int { // attempt (3)
if (n == 0) {
return 1;
} else if (n < 0) {
return 0;
}
auto find = memo.find(n);
if (find != memo.end()) {
return find->second;
}
//int num_steps = recurse(n - 1) + recurse(n - 2) + recurse(n - 3); // attempt (1) or (2)
int num_steps = recurse(memo, n - 1) + recurse(memo, n - 2) + recurse(memo, n - 3); // attempt (3)
memo[n] = num_steps;
return num_steps;
};
//return recurse(n); // attempt (1) or (2)
return recurse(memo, n); // attempt (3)
}
I have 3 slightly different attempts interleaved above but I cannot get any to compile. Is what I'm trying to do, possible?
I'm using clang on MacOS:
Apple LLVM version 10.0.0 (clang-1000.10.44.4)
Target: x86_64-apple-darwin18.2.0
Thread model: posix
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