How to convert recursive tree search function to nonrecursively? - c++

Question

I'm trying to explore the binary tree.
However, I have to implement recursive functions nonrecursively.
I've searched several ways to convert recursion to nonrecursive.
But it doesn't seem to apply to my code.
I wonder if I can convert my code to non-recursive and how I can convert it.
This is my code(recursive function)
const NODE* getNN(const float pos[DIM], const NODE* cur, int depth) {
if (!cur) return nullptr;
bool flag = pos[depth % DIM] < cur->pos[depth % DIM];
NODE* next{ flag ? cur->left : cur->right };
NODE* other{ flag ? cur->right : cur->left };
const NODE* temp = getNN(pos, next, depth + 1);
const NODE* best{ closest(pos, temp, cur) };
float r = this->distance(pos, best->pos);
float dr = pos[depth % DIM] - cur->pos[depth % DIM];
if (r >= dr * dr) {
temp = getNN(pos, other, depth + 1);
best = closest(pos, temp, best);
}
return best;
}
Here is what I expected
const NODE* getNN_NonRecur(const float pos[DIM])

Answer 1

It's been resolved it. Thank you for advice.
const NODE* getNN_NR(const float pos[DIM])
{
std::stack<std::pair<std::pair<NODE*, NODE*>, unsigned int>> st;
composeStack(st, pos, this->root, 0);
const NODE* best{ st.top().first.first };
while (!st.empty())
{
auto e = st.top(); st.pop();
if (!e.first.first) continue;
best = closest(pos, best, e.first.first);
float r = distance(pos, best->pos);
float dr = pos[e.second % DIM] - e.first.first->pos[e.second % DIM];
if (r >= dr * dr) {
composeStack(st, pos, e.first.second, e.second);
best = closest(pos, st.top().first.first, best);
}
}
return best;
}
void composeStack(std::stack<std::pair<std::pair<NODE*, NODE*>, unsigned int>>& st,
const float pos[DIM],
NODE* node, unsigned int depth)
{
NODE* cur = node;
st.push({ {node, nullptr}, depth });
while (cur) {
auto e = st.top();
cur = e.first.first; depth = e.second;
bool flag = pos[depth % DIM] < cur->pos[depth % DIM];
NODE* next{ flag ? cur->left : cur->right };
NODE* other{ flag ? cur->right : cur->left };
st.push(std::pair<std::pair
<NODE*, NODE*>, unsigned int>({ next, other }, depth + 1));
cur = next;
}
}