[LeetCode] Add daily challenge.

solved/LeetCode/Challenges/2020/January/09.kt

@@ -0,0 +1,37 @@
+class Solution {
+    fun ladderLength(beginWord: String, endWord: String, wordList: List<String>): Int {
+        if (endWord !in wordList) {
+            return 0
+        }
+        val valid = wordList.toSet()
+        val inf = 10000
+        val dist = mutableMapOf<String, Int>()
+        for (w in wordList) {
+            dist[w] = inf
+        }
+        dist[beginWord] = 1
+        val q = ArrayDeque<CharArray>()
+        q.add(beginWord.toCharArray())
+        val target = endWord.toCharArray()
+        while (q.isNotEmpty()) {
+            val cur = q.removeFirst()
+            val st = String(cur)
+            val newDist = dist[st]!! + 1
+            for (i in cur.indices) {
+                val next = cur.copyOf()
+                for (c in 'a'..'z') {
+                    next[i] = c
+                    if (next contentEquals target) {
+                        return newDist
+                    }
+                    val stNext = String(next)
+                    if (stNext in valid && newDist < dist.getOrDefault(stNext, inf)) {
+                        dist[stNext] = newDist
+                        q.add(next.copyOf())
+                    }
+                }
+            }
+        }
+        return 0
+    }
+}