[LeetCode] Add January 20.

solved/LeetCode/Challenges/2020/January/20.kt

@@ -0,0 +1,24 @@
+class Solution {
+  var state = 0
+  val matcher = mapOf(
+    '(' to ')',
+    '{' to '}',
+    '[' to ']'
+  )
+
+  fun isValid(s: String): Boolean {
+    return parse(s) && state == s.length
+  }
+
+  fun parse(s: String): Boolean {
+    if (state >= s.length) return true
+    val cur = s[state]
+    if (cur !in matcher.keys) return true
+    state++
+    val next = parse(s)
+    if (!next || state >= s.length) return false
+    if (matcher[cur] != s[state]) return false;
+    state++
+    return parse(s)
+  }
+}