[reeseo3o] WEEK 11 Solutions

missing-number/reeseo3o.js

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+// Time Complexity: O(n)
+// Space Complexity: O(1)
+
+const missingNumber = (nums) => {
+  const n = nums.length;
+  const total = (n * (n + 1)) / 2; // 가우스 공식
+  const sum = nums.reduce((acc, cur) => acc + cur, 0);
+  return total - sum;
+};

reorder-list/reeseo3o.js

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+// Time Complexity: O(n)
+// Space Complexity: O(1)
+
+const reorderList = (head) => {
+    if (!head || !head.next || !head.next.next) return;
+
+    // 1. 가운데 찾기 (slow는 앞쪽 리스트의 끝)
+    let slow = head;
+    let fast = head;
+    while (fast && fast.next) {
+      slow = slow.next;
+      fast = fast.next.next;
+    }
+
+    // 2. 뒷부분 뒤집기 (slow.next ~ 끝)
+    let prev = null;
+    let curr = slow.next;
+    slow.next = null; // 앞부분과 뒷부분 분리
+
+    while (curr) {
+      const next = curr.next;
+      curr.next = prev;
+      prev = curr;
+      curr = next;
+    }
+    // prev: 뒤집힌 두 번째 리스트의 head
+
+    // 3. 교차 머지 (head: 첫 번째, prev: 두 번째)
+    let first = head;
+    let second = prev;
+
+    while (second) {
+      const nextFirst = first.next;
+      const nextSecond = second.next;
+
+      first.next = second;
+      second.next = nextFirst;
+
+      first = nextFirst;
+      second = nextSecond;
+    }
+  };