algorithm: binary lifting

Graphs/BinaryLifting.js

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+/**
+ * Author: Adrito Mukherjee
+ * Binary Lifting implementation in Javascript
+ * Binary Lifting is a technique that is used to find the kth ancestor of a node in a rooted tree with N nodes
+ * The technique requires preprocessing the tree in O(N log N) using dynamic programming
+ * The techniqe can answer Q queries about kth ancestor of any node in O(Q log N)
+ * It is faster than the naive algorithm that answers Q queries with complexity O(Q K)
+ * It can be used to find Lowest Common Ancestor of two nodes in O(log N)
+ * Tutorial on Binary Lifting: https://codeforces.com/blog/entry/100826
+ */
+
+class BinaryLifting {
+  constructor (root, tree) {
+    this.connections = {}
+    this.up = {} // up[node][i] stores the 2^i-th  parent of node
+    for (const [i, j] of tree) {
+      this.addEdge(i, j)
+    }
+    // LOG should be such that 2^LOG is greater than total number of nodes in the tree
+    this.LOG = 0
+    while ((1 << this.LOG) <= Object.keys(this.connections).length) {
+      this.LOG++
+    }
+    this.dfs(root, root)
+  }
+
+  addNode (node) {
+    // Function to add a node to the tree (connection represented by set)
+    this.connections[node] = new Set()
+  }
+
+  addEdge (node1, node2) {
+    // Function to add an edge (adds the node too if they are not present in the tree)
+    if (!(node1 in this.connections)) {
+      this.addNode(node1)
+    }
+    if (!(node2 in this.connections)) {
+      this.addNode(node2)
+    }
+    this.connections[node1].add(node2)
+    this.connections[node2].add(node1)
+  }
+
+  dfs (node, parent) {
+    this.up[node] = {}
+    this.up[node][0] = parent
+    for (let i = 1; i < this.LOG; i++) {
+      this.up[node][i] = this.up[this.up[node][i - 1]][i - 1]
+    }
+    for (const child of this.connections[node]) {
+      if (child !== parent) this.dfs(child, node)
+    }
+  }
+
+  kthAncestor (node, k) {
+    for (let i = 0; i < this.LOG; i++) {
+      if (k & (1 << i)) {
+        node = this.up[node][i]
+      }
+    }
+    return node
+  }
+}
+
+function binaryLifting (root, tree, queries) {
+  const graphObject = new BinaryLifting(root, tree)
+  const ancestors = []
+  for (const [node, k] of queries) {
+    const ancestor = graphObject.kthAncestor(node, k)
+    ancestors.push(ancestor)
+  }
+  return ancestors
+}
+
+export { binaryLifting }
+
+// binaryLifting(
+//   0,
+//   [
+//     [0, 1],
+//     [0, 3],
+//     [0, 5],
+//     [5, 6],
+//     [1, 2],
+//     [1, 4],
+//     [4, 7],
+//     [7, 11],
+//     [7, 8],
+//     [8, 9],
+//     [9, 10]
+//   ],
+//   [
+//     [10, 4],
+//     [10, 7],
+//     [7, 2],
+//     [11, 3]
+//   ]
+// )
+
+// [4, 0, 1, 1]

Graphs/test/BinaryLifting.test.js

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+import { binaryLifting } from '../BinaryLifting'
+
+// The graph for Test Case 1 looks like this:
+//
+//         0
+//        /|\
+//       / | \
+//      1  3  5
+//     / \     \
+//    2   4     6
+//         \
+//          7
+//         / \
+//        11  8
+//             \
+//              9
+//               \
+//                10
+
+test('Test case 1', () => {
+  const root = 0
+  const graph = [
+    [0, 1],
+    [0, 3],
+    [0, 5],
+    [5, 6],
+    [1, 2],
+    [1, 4],
+    [4, 7],
+    [7, 11],
+    [7, 8],
+    [8, 9],
+    [9, 10]
+  ]
+  const queries = [
+    [2, 1],
+    [6, 1],
+    [7, 2],
+    [8, 2],
+    [10, 2],
+    [10, 3],
+    [10, 5],
+    [11, 3]
+  ]
+  const kthAncestors = binaryLifting(root, graph, queries)
+  expect(kthAncestors).toEqual([1, 5, 1, 4, 8, 7, 1, 1])
+})
+
+// The graph for Test Case 2 looks like this:
+//
+//         0
+//        / \
+//       1   2
+//      / \   \
+//     3   4   5
+//    /       / \
+//   6       7   8
+
+test('Test case 2', () => {
+  const root = 0
+  const graph = [
+    [0, 1],
+    [0, 2],
+    [1, 3],
+    [1, 4],
+    [2, 5],
+    [3, 6],
+    [5, 7],
+    [5, 8]
+  ]
+  const queries = [
+    [2, 1],
+    [3, 1],
+    [3, 2],
+    [6, 2],
+    [7, 3],
+    [8, 2],
+    [8, 3]
+  ]
+  const kthAncestors = binaryLifting(root, graph, queries)
+  expect(kthAncestors).toEqual([0, 1, 0, 1, 0, 2, 0])
+})