[alphaorderly] WEEK 07 Solutions

longest-substring-without-repeating-characters/alphaorderly.py

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+"""
+Time Complexity: O(n)
+Space Complexity: O(n)
+
+Approach:
+- Use a defaultdict to track the count of each character in the current window.
+- Maintain two pointers, 'left' and 'right', to represent the sliding window over the string.
+- As we iterate over the string with 'right', increment the count for the current character.
+- If a duplicate character appears in the window (count > 1), move the 'left' pointer forward and decrement counts until there are no duplicates.
+- After adjusting, update 'ans' with the maximum length found for a window with all unique characters.
+"""
+class Solution:
+    def lengthOfLongestSubstring(self, s: str) -> int:
+        count = defaultdict(int)
+        ans = left = 0
+
+        for right, val in enumerate(s):
+            count[val] += 1
+
+            while count[val] > 1:
+                count[s[left]] -= 1
+                left += 1
+
+            ans = max(ans, right - left + 1)
+
+        return ans

number-of-islands/alphaorderly.py

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+"""
+Time Complexity: O(m * n)
+Space Complexity: O(m * n)
+
+### BFS Approach ###
+
+Approach:
+- Use BFS to traverse all parts of each island in the grid.
+- Employ a queue to process all adjacent land cells iteratively.
+- Use a 'bound' helper function to check if a cell is within the grid bounds.
+- In 'island_marker', mark visited '1's with '#' to avoid revisiting.
+- For every cell in the grid, when a land cell ('1') is encountered, initiate BFS and increment the island count.
+"""
+class Solution:
+    def numIslands(self, grid: List[List[str]]) -> int:
+        DIR = [[0, 1], [1, 0], [-1, 0], [0, -1]]
+        ROW = len(grid)
+        COL = len(grid[0])
+
+        def bound(row: int, col: int) -> bool:
+            return 0 <= row < ROW and 0 <= col < COL
+
+        def island_marker(row: int, col: int) -> None:
+            q = deque([(row, col)])
+            grid[row][col] = "#"
+
+            while q:
+                r, c = q.popleft()
+
+                for dr, dc in DIR:
+                    tr, tc = r + dr, c + dc
+
+                    if not bound(tr, tc) or grid[tr][tc] != "1":
+                        continue
+
+                    grid[tr][tc] = "#"
+                    q.append((tr, tc))
+
+        ans = 0
+
+        for r in range(ROW):
+            for c in range(COL):
+                if grid[r][c] == "1":
+                    island_marker(r, c)
+                    ans += 1
+
+        return ans
+
+"""
+Time Complexity: O(m * n)
+Space Complexity: O(m * n)
+
+### DFS Approach ###
+
+Approach:
+- Use DFS to traverse all parts of each island in the grid.
+- Visitation is done recursively rather than with a stack, so stack comment is removed for clarity.
+- Use a 'bound' helper function to check if a cell is within the grid bounds.
+- In 'island_marker', mark visited '1's with '#' to avoid revisiting.
+- For every cell in the grid, when a land cell ('1') is encountered, initiate DFS and increment the island count.
+"""
+class Solution:
+    def numIslands(self, grid: List[List[str]]) -> int:
+        DIR = [[0, 1], [1, 0], [-1, 0], [0, -1]]
+        ROW = len(grid)
+        COL = len(grid[0])
+
+        def bound(row: int, col: int) -> bool:
+            return 0 <= row < ROW and 0 <= col < COL
+
+        def island_marker(row: int, col: int) -> None:
+            grid[row][col] = '#'
+
+            for dr, dc in DIR:
+                tr, tc = row + dr, col + dc
+                if bound(tr, tc) and grid[tr][tc] == '1':
+                    island_marker(tr, tc)
+
+        ans = 0
+
+        for r in range(ROW):
+            for c in range(COL):
+                if grid[r][c] == "1":
+                    island_marker(r, c)
+                    ans += 1
+
+        return ans

reverse-linked-list/alphaorderly.py

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+"""
+Time Complexity: O(n)
+Space Complexity: O(1)
+
+- We use a while loop to traverse the linked list.
+- We use a prev pointer to store the previous node.
+- We use a head pointer to store the current node.
+- We use a old_next pointer to store the next node.
+- We use a prev, head = head, old_next to update the prev and head pointers.
+- We return the prev pointer.
+"""
+
+class ListNode:
+    def __init__(self, val=0, next=None):
+        self.val = val
+        self.next = next
+
+class Solution:
+    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        prev = None
+
+        while head:
+            prev, head.next, head = head, prev, head.next
+
+        return prev

set-matrix-zeroes/alphaorderly.py

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+"""
+Time Complexity: O(m * n)
+Space Complexity: O(1)
+
+Approach:
+- Use the first row and first column as markers to track which rows and columns should be zeroed.
+- First, check if the original first row or first column should be zeroed by scanning them separately.
+- Then, scan the rest of the matrix. If an element is zero, set its corresponding first row and first column positions to zero.
+- Next, iterate through the matrix (excluding the first row and column) and set elements to zero if their corresponding first row or first column are zero.
+- Finally, zero the first row and/or first column if initially flagged.
+"""
+class Solution:
+    def setZeroes(self, matrix: List[List[int]]) -> None:
+        ROW = len(matrix)
+        COL = len(matrix[0])
+
+        row_check = any(matrix[0][c] == 0 for c in range(COL))
+        col_check = any(matrix[r][0] == 0 for r in range(ROW))
+
+        for r in range(ROW):
+            for c in range(COL):
+                if matrix[r][c] == 0:
+                    matrix[r][0] = 0
+                    matrix[0][c] = 0
+
+        for r in range(1, ROW):
+            for c in range(1, COL):
+                if matrix[r][0] == 0 or matrix[0][c] == 0:
+                    matrix[r][c] = 0
+
+        if row_check:
+            for c in range(COL):
+                matrix[0][c] = 0
+
+        if col_check:
+            for r in range(ROW):
+                matrix[r][0] = 0

unique-paths/alphaorderly.py

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+"""
+Time Complexity: O(m * n)
+Space Complexity: O(m * n)
+
+Dynamic Programming (2D DP approach):
+- Use a 2D array where maze[i][j] represents the number of unique paths to cell (i, j).
+- Initialize the first row and first column with 1 (since there's only one way to reach each cell: only right moves for the first row or only down moves for the first column).
+- For all other cells, maze[i][j] = maze[i-1][j] + maze[i][j-1] (sum of paths from the cell above and the cell to the left).
+- Return maze[m-1][n-1] as the answer, which is the total number of unique paths.
+"""
+class Solution:
+    def uniquePaths(self, m: int, n: int) -> int:
+        maze = [[0 for _ in range(n)] for _ in range(m)]
+
+        for i in range(m):
+            for j in range(n):
+                if i == 0 or j == 0:
+                    maze[i][j] = 1
+                else:
+                    maze[i][j] = maze[i-1][j] + maze[i][j-1]
+
+        return maze[m-1][n-1]
+
+"""
+Time Complexity: O(m * n)
+Space Complexity: O(n)
+
+Dynamic Programming (1D DP optimization):
+- Use a 1D array dp of size n.
+- dp[c] keeps track of the number of unique paths to column c in the current row.
+- Initialize dp with 1s (the first row has only one way to reach each column).
+- For every row from the second onward, update dp[c] = dp[c] + dp[c - 1] (add ways from the left neighbor to ways accumulated so far).
+- Return dp[-1] as the answer, representing the number of unique paths to the bottom-right cell.
+"""
+class Solution:
+    def uniquePaths(self, m: int, n: int) -> int:
+        dp = [1] * n
+
+        for _ in range(1, m):
+            for c in range(1, n):
+                dp[c] += dp[c - 1]
+
+        return dp[-1]
+
+"""
+Time Complexity: O(m + n)
+Space Complexity: O(1)
+
+Combinatorial approach:
+- The problem reduces to choosing (m-1) moves down from (m+n-2) total movements (or equivalently (n-1) moves right).
+- The number of unique paths is given by the formula (m+n-2)! / [(m-1)! * (n-1)!], representing all possible orderings of down and right moves.
+- Use the combinatorial (factorial) formula to compute the result efficiently.
+"""
+class Solution:
+    def uniquePaths(self, m: int, n: int) -> int:
+        return comb(m + n - 2, n - 1)