[LeetCode] Add January 19.

solved/LeetCode/Challenges/2020/January/19.kt

@@ -0,0 +1,37 @@
+class Solution {
+  private var seen = Array(1001) { Array(1001) {-1} }
+
+  private fun isPalindrome(s: String, i: Int, j: Int): Boolean {
+    if (j <= i) {
+      return true
+    }
+    if (seen[i][j] != -1) {
+      return seen[i][j] == 1
+    }
+    if (s[i] == s[j] && isPalindrome(s, i + 1, j - 1)) {
+      seen[i][j] = 1
+      return true
+    }
+    seen[i][j] = 0
+    return false
+  }
+
+  fun longestPalindrome(s: String): String {
+    var maxLen = 0
+    for (i in s.indices) {
+      for (j in i until s.length) {
+        if (isPalindrome(s, i, j)) {
+          maxLen = maxLen.coerceAtLeast(j - i)
+        }
+      }
+    }
+   for (i in s.indices) {
+      for (j in i until s.length) {
+        if (isPalindrome(s, i, j) && (j - i) == maxLen) {
+          return s.substring(i, j + 1)
+        }
+      }
+    }
+   error("answer not found.")
+  }
+}