[bhyun-kim, 김병현] Week 13 Solutions

Author: bhyun-kim

insert-interval/bhyun-kim.py

@@ -0,0 +1,45 @@
+"""
+57. Insert Interval
+https://leetcode.com/problems/insert-interval/
+
+Solution:
+    To solve this problem, we can follow the following steps:
+    1. Create an empty list called result to store the final intervals.
+    2. Initialize a variable i to 0 to iterate through the intervals.
+    3. Iterate through the intervals and add all intervals ending before the new interval starts to the result list.
+
+Time Complexity: O(n)
+    -
+
+"""
+
+
+from typing import List
+
+
+class Solution:
+    def insert(
+        self, intervals: List[List[int]], newInterval: List[int]
+    ) -> List[List[int]]:
+        result = []
+        i = 0
+        n = len(intervals)
+
+        # Add all intervals ending before the new interval starts
+        while i < n and intervals[i][1] < newInterval[0]:
+            result.append(intervals[i])
+            i += 1
+
+        # Merge all overlapping intervals with the new interval
+        while i < n and intervals[i][0] <= newInterval[1]:
+            newInterval[0] = min(newInterval[0], intervals[i][0])
+            newInterval[1] = max(newInterval[1], intervals[i][1])
+            i += 1
+        result.append(newInterval)
+
+        # Add all intervals starting after the new interval ends
+        while i < n:
+            result.append(intervals[i])
+            i += 1
+
+        return result

meeting-rooms-ii/bhyun-kim.py

@@ -0,0 +1,46 @@
+"""
+253. Meeting Rooms II
+https://leetcode.com/problems/meeting-rooms-ii/
+
+Solution:
+    To solve this problem, we can follow the following steps:
+    1. Sort the intervals based on their start times.
+    2. Initialize a list called room_times with a dummy interval.
+    3. Iterate through the intervals and assign each meeting to a room.
+    4. If a meeting can be assigned to an existing room, update the room's end time.
+    5. If a meeting cannot be assigned to an existing room, add a new room.
+    6. Return the number of rooms used.
+
+Time Complexity: O(nlogn)
+    - Sorting the intervals takes O(nlogn) time.
+    - Iterating through the intervals takes O(n) time.
+    - Overall, the time complexity is O(nlogn).
+
+Space Complexity: O(n)
+    - We are using a list to store the room times.
+"""
+
+from typing import List
+
+
+class Solution:
+    def minMeetingRooms(self, intervals: List[List[int]]) -> int:
+        intervals = sorted(intervals)
+        room_times = [[-1, -1]]
+        meet_idx = 0
+
+        while meet_idx < len(intervals):
+            m_t = intervals[meet_idx]
+            found_room = False
+            for i in range(len(room_times)):
+                r_t = room_times[i]
+                if m_t[0] >= r_t[1]:
+                    room_times[i] = m_t
+                    found_room = True
+                    break
+
+            if not found_room:
+                room_times.append(m_t)
+            meet_idx += 1
+
+        return len(room_times)

merge-intervals/bhyun-kim.py

@@ -0,0 +1,40 @@
+"""
+56. Merge Intervals
+https://leetcode.com/problems/merge-intervals/
+
+Solution:
+    To solve this problem, we can follow the following steps:
+    1. Sort the intervals by their start times.
+    2. Initialize an empty list called merged to store the final merged intervals.
+    3. Iterate through the sorted intervals and merge the current interval with the previous interval if there is an overlap.
+
+Time Complexity: O(nlogn)
+    - Sorting the intervals takes O(nlogn) time.
+    - Merging the intervals takes O(n) time.
+    - Overall, the time complexity is O(nlogn).
+
+Space Complexity: O(n)
+    - The space complexity is O(n) to store the merged intervals.
+"""
+
+from typing import List
+
+
+class Solution:
+    def merge(self, intervals: List[List[int]]) -> List[List[int]]:
+        if not intervals:
+            return []
+
+        # Sort intervals by their start times
+        intervals.sort(key=lambda x: x[0])
+
+        merged = []
+        for interval in intervals:
+            # if the list of merged intervals is empty or if the current interval does not overlap with the previous
+            if not merged or merged[-1][1] < interval[0]:
+                merged.append(interval)
+            else:
+                # there is an overlap, so we merge the current and previous intervals
+                merged[-1][1] = max(merged[-1][1], interval[1])
+
+        return merged

non-overlapping-intervals/bhyun-kim.py

@@ -0,0 +1,42 @@
+"""
+435. Non-overlapping Intervals
+https://leetcode.com/problems/non-overlapping-intervals/
+
+Solution:
+    To solve this problem, we can follow the following steps:
+    1. Sort the intervals based on their end times.
+    2. Initialize the end time of the last added interval.
+    3. Iterate through the intervals and add non-overlapping intervals to the count.
+    4. The number of intervals to remove is the total number minus the count of non-overlapping intervals.
+
+Time Complexity: O(nlogn)
+    - Sorting the intervals takes O(nlogn) time.
+    - Iterating through the intervals takes O(n) time.
+    - Overall, the time complexity is O(nlogn).
+
+Space Complexity: O(1)
+    - We are using a constant amount of space to store the end time of the last added interval.
+"""
+
+from typing import List
+
+
+class Solution:
+    def eraseOverlapIntervals(self, intervals: List[List[int]]) -> int:
+        if not intervals:
+            return 0
+
+        # Sort intervals based on end times
+        intervals.sort(key=lambda x: x[1])
+
+        # Initialize the end time of the last added interval
+        end = intervals[0][1]
+        count = 1
+
+        for i in range(1, len(intervals)):
+            if intervals[i][0] >= end:
+                count += 1
+                end = intervals[i][1]
+
+        # The number of intervals to remove is the total number minus the count of non-overlapping intervals
+        return len(intervals) - count

rotate-image/bhyun-kim.py

@@ -0,0 +1,48 @@
+"""
+48. Rotate Image
+https://leetcode.com/problems/rotate-image/
+
+Solution:
+    To solve this problem, we can follow the following steps:
+    1. Process layers from the outermost to the innermost.
+    2. For each layer, iterate through the elements in the layer.
+    3. Swap the elements in the layer in a clockwise direction.
+    4. Repeat the process for all layers.
+
+Time Complexity: O(n^2)
+    - We need to process all elements in the matrix.
+    - The time complexity is O(n^2).
+
+Space Complexity: O(1)
+    - We are rotating the matrix in place without using any extra space.
+"""
+
+from typing import List
+
+
+class Solution:
+    def rotate(self, matrix: List[List[int]]) -> None:
+        """
+        Do not return anything, modify matrix in-place instead.
+        """
+        n = len(matrix)
+        # Process layers from the outermost to the innermost
+        for layer in range(n // 2):
+            first = layer
+            last = n - layer - 1
+            for i in range(first, last):
+                offset = i - first
+                # Save the top element
+                top = matrix[first][i]
+
+                # Move left element to top
+                matrix[first][i] = matrix[last - offset][first]
+
+                # Move bottom element to left
+                matrix[last - offset][first] = matrix[last][last - offset]
+
+                # Move right element to bottom
+                matrix[last][last - offset] = matrix[i][last]
+
+                # Assign top element to right
+                matrix[i][last] = top