From d533a1774f62799b51b3c9c37d9633f0a811cece Mon Sep 17 00:00:00 2001
From: Matej Focko <mfocko@redhat.com>
Date: Sun, 14 Aug 2022 21:19:49 +0200
Subject: [PATCH] =?UTF-8?q?problems:=20add=20=E2=80=9E126.=20Word=20Ladder?=
 =?UTF-8?q?=20II=E2=80=9C?=
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Signed-off-by: Matej Focko <mfocko@redhat.com>
---
 problems/word-ladder-ii.kt | 157 +++++++++++++++++++++++++++++++++++++
 1 file changed, 157 insertions(+)
 create mode 100644 problems/word-ladder-ii.kt

diff --git a/problems/word-ladder-ii.kt b/problems/word-ladder-ii.kt
new file mode 100644
index 0000000..ba22903
--- /dev/null
+++ b/problems/word-ladder-ii.kt
@@ -0,0 +1,157 @@
+class Solution {
+    private fun wordDifference(lhs: String, rhs: String): Int = lhs.zip(rhs).count { (l, r) -> l != r }
+
+    private enum class VertexState {
+        Unvisited, Enqueued, Done
+    }
+
+    private data class Vertex<T>(
+        val label: T, val neighbours: MutableSet<Vertex<T>> = mutableSetOf()
+    ) {
+        var distance: Int = Int.MAX_VALUE
+        val parents: MutableSet<Vertex<T>> = mutableSetOf()
+        var state: VertexState = VertexState.Unvisited
+            private set
+
+        fun add(neighbour: Vertex<T>) {
+            neighbours.add(neighbour)
+        }
+
+        fun addParent(parent: Vertex<T>) {
+            parents.add(parent)
+        }
+
+        fun transition() {
+            state = when (state) {
+                VertexState.Unvisited -> VertexState.Enqueued
+                VertexState.Enqueued -> VertexState.Done
+                VertexState.Done -> error("Cannot transition further!")
+            }
+        }
+
+        override fun hashCode(): Int = label.hashCode()
+        override fun toString(): String = "$label(${neighbours.map { it.label.toString() }})"
+        override fun equals(other: Any?): Boolean {
+            if (this === other) return true
+            if (javaClass != other?.javaClass) return false
+
+            other as Vertex<*>
+
+            if (label != other.label) return false
+
+            return true
+        }
+    }
+
+    private class Graph<T> {
+        private val vertices: MutableMap<T, Vertex<T>> = mutableMapOf()
+
+        fun addVertex(src: T, dst: T) {
+            val u = vertices.getOrPut(src) { Vertex(src) }
+            val v = vertices.getOrPut(dst) { Vertex(dst) }
+
+            u.add(v)
+        }
+
+        fun addSymmetricVertex(src: T, dst: T) {
+            addVertex(src, dst)
+            addVertex(dst, src)
+        }
+
+        private fun processVertex(queue: ArrayDeque<Vertex<T>>, u: Vertex<T>) {
+            u.neighbours.forEach { v ->
+                if (v.state == VertexState.Unvisited) {
+                    queue.addLast(v)
+
+                    queue.last().distance = u.distance + 1
+                    queue.last().transition()
+                } else if (u.distance + 1 != v.distance) {
+                    return@forEach
+                }
+
+                v.addParent(u)
+            }
+
+            u.transition()
+        }
+
+        fun bfs(src: T, dst: T) {
+            if (vertices[src] == null) {
+                return
+            }
+
+            val queue = ArrayDeque<Vertex<T>>()
+            queue.addLast(vertices[src]!!)
+
+            queue.last().distance = 0
+            queue.last().transition()
+
+            while (queue.isNotEmpty()) {
+                val u = queue.removeFirst()
+                if (u.label == dst) {
+                    break
+                }
+
+                processVertex(queue, u)
+            }
+        }
+
+        fun parents(vertex: T): List<T> = vertices[vertex]?.parents?.map(Vertex<T>::label) ?: emptyList()
+
+        override fun toString(): String = vertices.toString()
+    }
+
+    private fun constructGraph(src: String, vertices: List<String>): Graph<String> {
+        val g = Graph<String>()
+
+        vertices
+            .filter { wordDifference(src, it) == 1 }
+            .forEach { g.addVertex(src, it) }
+
+        vertices.indices
+            .flatMap { listOf(it).zip(it + 1 until vertices.size) }
+            .map { (i, j) -> vertices[i] to vertices[j] }
+            .filter { (lhs, rhs) -> wordDifference(lhs, rhs) == 1 }
+            .forEach { (lhs, rhs) -> g.addSymmetricVertex(lhs, rhs) }
+
+        return g
+    }
+
+    private fun constructPath(
+        graph: Graph<String>, currentPath: MutableList<String>, paths: MutableList<MutableList<String>>
+    ): MutableList<MutableList<String>> {
+        if (graph.parents(currentPath.last()).isEmpty()) {
+            paths.add(currentPath.reversed().toMutableList())
+            return paths
+        }
+
+        for (parent in graph.parents(currentPath.last()).filter { !currentPath.contains(it) }) {
+            currentPath.add(parent)
+            constructPath(graph, currentPath, paths)
+            currentPath.remove(parent)
+        }
+
+        return paths
+    }
+
+    fun findLadders(
+        beginWord: String, endWord: String, wordList: List<String>
+    ): List<List<String>> {
+        val graph = constructGraph(beginWord, wordList)
+        graph.bfs(beginWord, endWord)
+        return constructPath(graph, mutableListOf(endWord), mutableListOf()).filter { it.count() >= 2 }
+    }
+}
+
+fun main() {
+    val s = Solution()
+
+    check(
+        s.findLadders("hit", "cog", listOf("hot", "dot", "dog", "lot", "log", "cog")) == listOf(
+            listOf("hit", "hot", "dot", "dog", "cog"), listOf("hit", "hot", "lot", "log", "cog")
+        )
+    )
+    check(s.findLadders("hit", "cog", listOf("hot", "dot", "dog", "lot", "log")) == emptyList<List<String>>())
+    check(s.findLadders("hot", "dog", listOf("hot", "dog")) == emptyList<List<String>>())
+    check(s.findLadders("hot", "dog", listOf("hot", "dog", "dot")) == listOf(listOf("hot", "dot", "dog")))
+}