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use super::rooting::TreeNode;
pub struct Tree {
pub root: TreeNode,
pub size: usize,
}
pub struct LcaSolver {
sparse_table: MinSparseTable,
node_order: Vec<usize>,
last: Vec<usize>,
}
impl LcaSolver {
pub fn new(tree: &Tree) -> Self {
let mut node_depth = vec![0usize; tree.size * 2 - 1];
let mut node_order = vec![0usize; tree.size * 2 - 1];
let mut last = vec![0usize; tree.size];
let mut tour_index = 0;
let mut visit = |node: usize, depth: usize| {
node_order[tour_index] = node;
node_depth[tour_index] = depth;
last[node] = tour_index;
tour_index += 1;
};
let mut stack = vec![(&tree.root, 0usize)];
let mut visited = vec![false; tree.size];
while let Some((node, depth)) = stack.pop() {
visit(node.id, depth);
if !visited[node.id] {
visited[node.id] = true;
for child in &node.children {
stack.push((node, depth));
stack.push((child, depth + 1));
}
}
}
let sparse_table = MinSparseTable::new(&node_depth);
Self {
sparse_table,
node_order,
last,
}
}
pub fn lca(&self, a: usize, b: usize) -> usize {
let (a, b) = (self.last[a], self.last[b]);
let (l, r) = if a < b { (a, b) } else { (b, a) };
let idx = self.sparse_table.query_index(l, r);
self.node_order[idx]
}
}
pub struct MinSparseTable {
min_depth: Vec<Vec<Option<usize>>>,
index: Vec<Vec<Option<usize>>>,
log2: Vec<usize>,
}
impl MinSparseTable {
pub fn new(node_depth: &[usize]) -> Self {
let n = node_depth.len();
let log2 = Self::build_log2(n);
let m = log2[n];
let mut min_depth = vec![vec![None; n]; m + 1];
let mut index = vec![vec![None; n]; m + 1];
for (i, &depth) in node_depth.iter().enumerate() {
min_depth[0][i] = Some(depth);
index[0][i] = Some(i);
}
for i in 1..=m {
for j in 0..=(n - (1 << i)) {
let left_interval = min_depth[i - 1][j];
let right_interval = min_depth[i - 1][j + (1 << (i - 1))];
if left_interval <= right_interval {
min_depth[i][j] = left_interval;
index[i][j] = index[i - 1][j];
} else {
min_depth[i][j] = right_interval;
index[i][j] = index[i - 1][j + (1 << (i - 1))];
}
}
}
Self {
min_depth,
index,
log2,
}
}
fn build_log2(n: usize) -> Vec<usize> {
let mut log2 = vec![0usize; n + 1];
for i in 2..=n {
log2[i] = log2[i / 2] + 1;
}
log2
}
fn query_index(&self, l: usize, r: usize) -> usize {
let len = r - l + 1;
let i = self.log2[len];
let left_interval = self.min_depth[i][l];
let right_interval = self.min_depth[i][r - (1 << i) + 1];
if left_interval <= right_interval {
self.index[i][l]
} else {
self.index[i][r - (i << i) + 1]
}
.unwrap()
}
}
#[cfg(test)]
mod tests {
use super::*;
use crate::algo::graph::UnweightedAdjacencyList;
#[test]
fn test_tree_lowest_commmon_ancestor() {
let tree = UnweightedAdjacencyList::new_undirected(
17,
&[
[0, 1],
[0, 2],
[1, 3],
[1, 4],
[2, 5],
[2, 6],
[2, 7],
[3, 8],
[3, 9],
[5, 10],
[5, 11],
[7, 12],
[7, 13],
[11, 14],
[11, 15],
[11, 16],
],
);
let size = tree.vertices_count();
let root = TreeNode::from_adjacency_list(&tree, 0);
let tree = Tree { root, size };
let lca_solver = LcaSolver::new(&tree);
assert_eq!(lca_solver.lca(14, 13), 2);
assert_eq!(lca_solver.lca(9, 11), 0);
assert_eq!(lca_solver.lca(12, 12), 12);
}
}