diff --git a/src/Utils.kt b/src/Utils.kt
index 87aec90..23dfa16 100644
--- a/src/Utils.kt
+++ b/src/Utils.kt
@@ -17,6 +17,14 @@ fun readInputAsCommaSeparatedInts(day: Int, name: String) = openFile(day, name)
     .split(",")
     .map { it.toInt() }
 
+fun readGraph(day: Int, name: String) = readInput(day, name).fold(mapOf<String, Set<String>>()) { currentGraph, edge ->
+    val (fromVertex, toVertex) = edge.split("-")
+    val fromNeighbours = currentGraph.getOrDefault(fromVertex, emptySet()) + toVertex
+    val toNeighbours = currentGraph.getOrDefault(toVertex, emptySet()) + fromVertex
+
+    currentGraph + mapOf(fromVertex to fromNeighbours, toVertex to toNeighbours)
+}.toMap()
+
 /**
  * Converts string to md5 hash.
  */
diff --git a/src/year2021/day12/Day12.kt b/src/year2021/day12/Day12.kt
new file mode 100644
index 0000000..d46e215
--- /dev/null
+++ b/src/year2021/day12/Day12.kt
@@ -0,0 +1,61 @@
+package year2021.day12
+
+import readGraph
+
+fun findAllPaths(graph: Map<String, Set<String>>, path: List<String>, visited: Set<String>): Set<String> {
+    if (path.last() == "end") {
+        return setOf(path.joinToString("-"))
+    }
+
+    val neighbours = graph
+        .getOrDefault(path.last(), emptySet())
+        .filter { it == it.uppercase() || !visited.contains(it) }
+
+    return neighbours.map { neighbour ->
+        findAllPaths(graph, path + neighbour, visited + neighbour)
+    }.fold(emptySet()) { acc, it -> acc + it }
+}
+
+fun part1(input: Map<String, Set<String>>): Int =
+    findAllPaths(input, listOf("start"), setOf("start")).size
+
+fun findAllPaths(
+    graph: Map<String, Set<String>>,
+    path: List<String>,
+    visited: Set<String>,
+    twiceVisited: Boolean
+): Set<String> {
+    if (path.last() == "end") {
+        return setOf(path.joinToString("-"))
+    }
+
+    val neighbours = graph
+        .getOrDefault(path.last(), emptySet())
+        .filter { it == it.uppercase() || (!twiceVisited && it != "start") || !visited.contains(it) }
+
+    return neighbours.map { neighbour ->
+        if (neighbour == neighbour.lowercase() && visited.contains(neighbour))
+            findAllPaths(graph, path + neighbour, visited, true)
+        else
+            findAllPaths(graph, path + neighbour, visited + neighbour, twiceVisited)
+    }.fold(emptySet()) { acc, it -> acc + it }
+}
+
+fun part2(input: Map<String, Set<String>>): Int = findAllPaths(input, listOf("start"), setOf("start"), false).size
+
+fun main() {
+    val sample = readGraph(12, "sample")
+    val slightlyLargerSample = readGraph(12, "slightly_larger_sample")
+    val evenLargerExample = readGraph(12, "even_larger_sample")
+    val input = readGraph(12, "input")
+
+    check(part1(sample) == 10)
+    check(part1(slightlyLargerSample) == 19)
+    check(part1(evenLargerExample) == 226)
+    println(part1(input))
+
+    check(part2(sample) == 36)
+    check(part2(slightlyLargerSample) == 103)
+    check(part2(evenLargerExample) == 3509)
+    println(part2(input))
+}
diff --git a/src/year2021/day12/even_larger_sample.txt b/src/year2021/day12/even_larger_sample.txt
new file mode 100644
index 0000000..da6e083
--- /dev/null
+++ b/src/year2021/day12/even_larger_sample.txt
@@ -0,0 +1,18 @@
+fs-end
+he-DX
+fs-he
+start-DX
+pj-DX
+end-zg
+zg-sl
+zg-pj
+pj-he
+RW-he
+fs-DX
+pj-RW
+zg-RW
+start-pj
+he-WI
+zg-he
+pj-fs
+start-RW
\ No newline at end of file
diff --git a/src/year2021/day12/sample.txt b/src/year2021/day12/sample.txt
new file mode 100644
index 0000000..898cd56
--- /dev/null
+++ b/src/year2021/day12/sample.txt
@@ -0,0 +1,7 @@
+start-A
+start-b
+A-c
+A-b
+b-d
+A-end
+b-end
\ No newline at end of file
diff --git a/src/year2021/day12/slightly_larger_sample.txt b/src/year2021/day12/slightly_larger_sample.txt
new file mode 100644
index 0000000..ef30b81
--- /dev/null
+++ b/src/year2021/day12/slightly_larger_sample.txt
@@ -0,0 +1,10 @@
+dc-end
+HN-start
+start-kj
+dc-start
+dc-HN
+LN-dc
+HN-end
+kj-sa
+kj-HN
+kj-dc
\ No newline at end of file