[okyungjin] WEEK 11 Solutions

merge-intervals/okyungjin.py

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+'''
+https://leetcode.com/problems/merge-intervals/description/
+
+N: len(intervals)
+Time: O(N logN), 정렬
+Space: O(N), answer 배열
+'''
+class Solution:
+    def merge(self, intervals: List[List[int]]) -> List[List[int]]:
+        intervals.sort(key=lambda x: x[0])
+
+        answer = [intervals[0]]
+
+        '''
+        [1,4], [5,8] # case A
+        [1,4], [4,7] # case B
+        [1,4], [2,8] # case C
+        '''
+
+        for i in range(1, len(intervals)):
+            curr_start, curr_end = intervals[i]
+            _, last_end = answer[-1]
+
+            if curr_start > last_end: # case A
+                answer.append(intervals[i])
+            elif curr_end > last_end: # case C
+                answer[-1][1] = curr_end
+
+        return answer

missing-number/okyungjin.py

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+'''
+https://leetcode.com/problems/missing-number/description/
+'''
+
+'''
+solution1: 정렬 후 이진탐색
+
+n: len(nums)
+Time: O(n log n), sort
+Space: O(1)
+'''
+class Solution:
+    def missingNumber(self, nums: List[int]) -> int:
+        nums.sort() # Time: O(n log n)
+
+        left = 0
+        right = len(nums)
+
+        while left < right: # Time: O(log n)
+            mid = (left + right) // 2
+
+            if nums[mid] > mid:
+                right = mid
+            else:
+                left = mid + 1
+
+        return left
+
+'''
+solution2: 0부터 n까지 더한 값에서 배열의 전체 합을 빼기
+
+n: len(nums)
+Time: O(n)
+Space: O(1)
+'''
+class Solution:
+    def missingNumber(self, nums: List[int]) -> int:
+        n = len(nums)
+
+        complete_sum = n * (n + 1) // 2
+        actual_sum = sum(nums) # Time: O(n)
+
+        return complete_sum - actual_sum

reorder-list/okyungjin.py

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+"""
+https://leetcode.com/problems/reorder-list/
+
+N: number of nodes
+Time: O(N)
+Space: O(N)
+"""
+class Solution:
+    def reorderList(self, head: Optional[ListNode]) -> None:
+        nodes = []
+        curr = head
+        while curr:
+            nodes.append(curr)
+            curr = curr.next
+
+        left = 0
+        right = len(nodes) - 1
+
+        while left < right:
+            nodes[left].next = nodes[right]
+            left += 1
+
+            if left == right:
+                break
+
+            nodes[right].next = nodes[left]
+            right -= 1
+
+        nodes[left].next = None