Sort List (Medium)

Divide & Conquer/Sort List (Medium)/max.js

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+// 1차 (PASS)
+// 시간복잡도: O(N * logN)
+// 공간복잡도: O(logN)
+// @see https://www.geeksforgeeks.org/merge-sort-for-linked-list/
+
+/**
+ * Definition for singly-linked list.
+ * function ListNode(val, next) {
+ *     this.val = (val===undefined ? 0 : val)
+ *     this.next = (next===undefined ? null : next)
+ * }
+ */
+/**
+ * @param {ListNode} head
+ * @return {ListNode}
+ */
+var sortList = function (head) {
+  return divideAndConquer(head);
+
+  function divideAndConquer(h) {
+    if (h === null || h.next === null) {
+      return h;
+    }
+
+    const center = getCenterHead(h);
+    const centerNext = center.next;
+    center.next = null;
+
+    const left = divideAndConquer(h);
+    const right = divideAndConquer(centerNext);
+
+    return sortedMerge(left, right);
+  }
+
+  function getCenterHead(h) {
+    if (h === null) {
+      return h;
+    }
+
+    let fast = h;
+    let slow = h;
+
+    while (fast.next !== null && fast.next.next !== null) {
+      slow = slow.next;
+      fast = fast.next.next;
+    }
+
+    return slow;
+  }
+
+  function sortedMerge(a, b) {
+    let result = null;
+
+    if (a === null) {
+      return b;
+    }
+    if (b === null) {
+      return a;
+    }
+
+    if (a.val < b.val) {
+      result = a;
+      result.next = sortedMerge(a.next, b);
+    } else {
+      result = b;
+      result.next = sortedMerge(a, b.next);
+    }
+
+    return result;
+  }
+};