eifueo/docs/2a/ece250.md

# ECE 250: DSA

## Solving recurrences

The **master method** is used to solve recurrences. For expressions of the form $T(n)=aT(n/b)+f(n)$:

- If $f(n)=O(n^{\log_b a})$, we have $T(n)=\Theta(n^{\log_b a}\log n)$
- If $f(n) < O(n^{\log_b a})$, we have $T(n)=O(n^{\log_b a})$
- If $f(n) > \Omega(n^{\log_b a})$, and $af(n/b)\leq cf(n), c<0$, we have $T(n)=\Theta(f(n))$


## Heaps

A heap is a binary tree **stored in an array** in which all levels but the lowest are filled. It is guaranteed that the parent of index $i$ is greater than or equal to the element at index $i$.

- the parent of index $i$ is stored at $i/2$
- the left child of index $i$ is stored at $2i$
- the right child of index $i$ is stored at $2i+1$


<img src="https://upload.wikimedia.org/wikipedia/commons/c/c4/Max-Heap-new.svg" width=600>(Source: Wikimedia Commons)</img>

The **heapify** command takes a node and makes it and its children a valid heap.

```rust
fn heapify(&mut A: Vec, i: usize) {
	if A[2*i] >= A[i] {
		A.swap(2*i, i);
		heapify(A, 2*i)
	} else if A[2*i + 1] >= A[i] {
		A.swap(2*i + 1, i);
		heapify(A, 2*i + 1)
	}
}
```

Repeatedly heapifying an array from middle to beginning converts it to a heap.

```rust
fn build_heap(A: Vec) {
	let n = A.len()
	for i in (n/2).floor()..0 {	// this is technically not valid but it's much clearer
		heapify(A, i);
	}
}
```

### Heapsort

Heapsort constructs a heap annd then does magic things that I really cannot be bothered to figure out right now.

```rust
fn heapsort(A: Vec) {
	build_heap(A);
	let n = A.len();
	for i in n..0 {
		A.swap(1, i);
		heapify(A, 1);	// NOTE: heapify takes into account the changed value of n
	}
}
```

### Priority queues

A priority queue is a heap with the property that it can remove the highest value in $O(\log n)$ time.

```rust
fn pop(A: Vec, &n: usize) {
	let biggest = A[0];
	
	A[0] = n;
	*n -= 1;
	heapify(A, 1);
	return biggest;
}
```

```rust
fn insert(A: Vec, &n: usize, key: i32) {
	*n += 1;
	
	let i = n;
	while i > 1 && A[parent(i)] < key {
		A[i] = A[parent(i)];
		i = parent(i);
	}
	A[i] = k;
}
```

## Sorting algorithms

### Quicksort

Quicksort operates by selecting a **pivot point** that ensures that everything to the left of the pivot is less than anything to the right of the pivot, which is what partitioning does.

```rust
fn partition(A: Vec, left_bound: usize, right_bound: usize) {
	let i = left_bound;
	let j = right_bound;
	
	while true {
		while A[j] <= A[right_bound] { j -= 1; }
		while A[i] >= A[left_bound] { i += 1; }
		
		if i < j { A.swap(i, j); }
		else { return j }  // new bound!
	}
}
```

Sorting calls partitioning with smaller and smaller bounds until the collection is sorted.

```rust
fn sort(a: Vec, left: usize, right: usize) {
	if left < right {
		let pivot = partition(A, left, right);
		sort(A, left, pivot);
		sort(A, pivot+1, right);
	}
}
```

- In the best case, if partitioning is even, the time complexity is $T(n)=T(n/2)+\Theta(n)=\Theta(n\log n)$.
- In the worst case, if one side only has one element, which occurs if the list is sorted, the time complexity is $\Theta(n^2)$.

### Counting sort

If items are or are linked to a number from $1..n$ (duplicates are allowed), counting sort counts the number of each number, then moves things to the correct position. Where $k$ is the size of the counter array, the time complexity is $O(n+k)$.

First, construct a count prefix sum array:

```rust
fn count(A: Vec, K: usize) {
	let counter = vec![0; K];
	
	for i in A {
		counter[i] += 1;
	}
	
	for (index, val) in counter.iter_mut().enumerate() {
		counter[index + 1] += val;   // ignore bounds for cleanliness please :)
	}
	return counter
}
```

Next, the prefix sum represents the correct position for each item.

```rust
fn sort(A: Vec) {
	let counter = count(A, 100);
	let sorted = vec![0; A.len()];
	
	for i in n..0 {
		sorted[counter[A[i]]] = A[i];
		counter[A[i]] -= 1;
	}
}
```

## Graphs

!!! definition
    - A **vertex** is a node.
    - The **degree** of a node is the number of edges connected to it.
    - A **connected graph** is such that there exists a path from any node in the graph to any other node.
    - A **connected component** is a subgraph such that there exists a path from any node in the subgraph to any other node in the subgraph.
    - A **tree** is a connected graph without cycles.

### Directed acyclic graphs

a DAG is acyclic if and only if there are no **back edges** — edges from a child to an ancestor.

### Bellman-Ford

The Bellman-Ford algorithm allows for negative edges and detects negative cycles.

```rust
fn bf(G: Graph, s: Node) {
	let mut distance = Vec::new(INFINITY);
	let mut adj_list = Vec::from(G);
	
	distance[s] = 0;
	
	for i in 1..G.vertices.len()-1 {
		for (u,v) in G.edges {
			if distance[v] > distance[u] + adj_list[u][v] {
				distance[v] = distance[u] + adj_list[u][v];
			}
		}
	}
	for (u, v) in G.edges {
		if distance[v] > distance[u] + adj_list[u][v] {
			return false;
		}
	}
	return true;
}
```

### Topological sort

This is used to find the shortest path in a DAG simply by DFS.

```rust
fn shortest_path(G: Graph, s: Node) {
	let nodes: Vec<Node> = top_sort(G);
	let mut adj_list = G.to_adj_list();
	let mut distance = Vec::new(INFINITY);
	
	for v in nodes {
		for adjacent in adj_list[v] {
			if distance[adjacent] > distance[v] + adjacent[v] {
				distance[v] = distance[adjacent] + adjacent[v];
			}
		}
	}
}
```

### Minimum spanning tree

!!! definition
    - A **cut** $(S, V-S)$ is a partition of vertices into disjoint sets $S$ and $V-S$.
    - An edge $u,v\in E$ **crosses the cut** $(S,V-S)$ if t`he endpoints are on different sides of the cut.
    - A cut **respects** a set of edges $A$ if and only if no edge in $A$ crosses the cut.
    - A **light edge** is the minimum of all edges that could cross the cut. There can be more than one light edge per cut.

A **spanning tree** of $G$ is a subgraph that contains all of its vertices. An MST minimises the sum of all edges in the spanning tree.

To create an MST:

1. Add edges from the spanning tree to an empty set, maintaining that the set is always a subset of an MST (only "safe edges" are added)

The **Prim-Jarnik algorithm** grows a tree one vertex at a time. $A$ is a subset of the already computed portion of $T$, and all vertices outside $A$ have a weight of infinity if there is no edge.

```rust
// r is the start vertex
fn create_mst_prim(G: Graph, r: Vertex) {
	// clean all vertices
	for vertex in G.vertices.iter_mut() {
		vertex.min_weight = INFINITY;
		vertex.parent = None;
	}
	
	let Q = BinaryHeap::from(G.vertices); // priority queue
	
	while let Some(u) = Q.pop() {
		for v in u.adjacent_vertices.iter_mut() {
			if Q.contains(v) && v.edge_to(u).weight < v.min_weight {
				v.min_weight = v.edge_to(u).weight;
				Q."modify_key"(v);
				v.parent = u;
			}
		}
	}
	
}
```

**Kruskal's algorithm** is objectively better by relying on edges instead.

```rust
fn create_mst_kruskal(G: Graph) -> HashSet<Edge> {
	let mut A = HashSet::new();
	let mut S = DisjointSet::new();  // vertices set
	
	for v in G.vertices.iter() {
		S.add_as_new_set(v);
	}
	G.edges.sort(|edge| edge.weight);
	
	for (from, dest) in G.edges {
		if S.find_set_that_contains(from) != S.find_set_that_contains(dest) {
			A.insert((from, to));
			let X = S.pop(from);
			let Y = S.pop(to);
			S.insert({X.union(Y)});
		}
	}
	return A;
}
```

The time complexity is $O(E\log V)$.

### All pairs shortest path

Also known as an adjacency matrix extended such that each point represents the minimum distance from one edge to that other edge.