[LTC] Lv2 / Linked List Random Node / 5분

leetcode/0chnxxx/Linked_List_Random_Node.kt

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+import java.util.Random
+
+/**
+ * Given a singly linked list, return a random node's value from the linked list.
+ * Each node must have the same probability of being chosen.
+ *
+ * Implement the Solution class:
+ * Solution(ListNode head) Initializes the object with the head of the singly-linked list head.
+ * int getRandom() Chooses a node randomly from the list and returns its value. All the nodes of the list should be equally likely to be chosen.
+ *
+ * Constraints:
+ * The number of nodes in the linked list will be in the range [1, 10^4].
+ * -10^4 <= Node.val <= 10^4
+ * At most 104 calls will be made to getRandom.
+ *
+ *
+ * Follow up:
+ * What if the linked list is extremely large and its length is unknown to you?
+ * Could you solve this efficiently without using extra space?
+ */
+
+fun main() {
+    val head = ListNode(1, ListNode(2, ListNode(3)))
+
+    val result = Solution(head).getRandom()
+
+    println(result)
+}
+
+class ListNode(
+    var `val`: Int,
+    var next: ListNode? = null
+)
+
+class Solution(head: ListNode?) {
+    private val values = mutableListOf<Int>()
+
+    // 시간 복잡도 : O(N)
+    // 공간 복잡도 : O(N)
+    init {
+        var current = head
+
+        while (current != null) {
+            values.add(current.`val`)
+            current = current.next
+        }
+    }
+
+    // 시간 복잡도 : O(1)
+    // 공간 복잡도 : O(N)
+    fun getRandom(): Int {
+        return values[Random().nextInt(values.size)]
+    }
+}