Completed Array-2

Problem1.py

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+# Time Complexity : O(n)  # We traverse array twice
+# Space Complexity : O(1) # Ignoring the result array, we only use variables in-place
+# Did this code successfully run on Leetcode : Yes
+# Approach :
+# 1. Use the values as indices: mark the index (num-1) as negative to say "this number exists".
+# 2. After marking, indices that remain positive were never visited.
+# 3. Those indices + 1 are the missing numbers.
+
+from typing import List
+
+class Solution:
+    def findDisappearedNumbers(self, nums: List[int]) -> List[int]:
+        n = len(nums)
+
+        # First pass: mark numbers as seen by making their index negative
+        for i in range(n):
+            idx = abs(nums[i]) - 1    # map number to index (1 → 0, 2 → 1, ... n → n-1)
+            if nums[idx] > 0:         # only flip if it's not already negative
+                nums[idx] *= -1       # mark index as visited
+
+        # Second pass: collect missing numbers
+        result = []
+        for i in range(n):
+            if nums[i] > 0:           # if still positive → index never marked → number missing
+                result.append(i + 1)  # convert back to 1-based number
+
+        return result

Problem2.py

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+# Time Complexity : O(n)
+# Space Complexity : O(1)
+# Did this code successfully run on Leetcode : Yes
+# Approach :
+# 1. Compare elements in pairs, reducing comparisons by half.
+# 2. First compare the two elements to decide smaller/larger.
+# 3. Update global min and max accordingly.
+
+def getMinMax(arr):
+    n = len(arr)
+
+    # If array has even number of elements
+    if n % 2 == 0:
+        if arr[0] < arr[1]:
+            mn, mx = arr[0], arr[1]
+        else:
+            mn, mx = arr[1], arr[0]
+        i = 2
+    else:
+        # If odd, initialize with first element
+        mn = mx = arr[0]
+        i = 1
+
+    # Process pairs
+    while i < n - 1:
+        if arr[i] < arr[i+1]:
+            mn = min(mn, arr[i])
+            mx = max(mx, arr[i+1])
+        else:
+            mn = min(mn, arr[i+1])
+            mx = max(mx, arr[i])
+        i += 2
+
+    return mn, mx

Problem3.py

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+# Time Complexity : O(m * n)
+# Space Complexity : O(1)   # done in-place, no extra grid used
+# Did this code successfully run on LeetCode : Yes
+# Any problem you faced while coding this : 
+#   Needed to carefully handle transitional states (1->0 and 0->1)
+
+
+class Solution:
+    def gameOfLife(self, board: List[List[int]]) -> None:
+        """
+        Do not return anything, modify board in-place instead.
+        """
+
+        # All 8 possible directions around a cell
+        dirs = [(-1, -1), (-1, 0), (-1, 1),(0, -1),(0, 1),(1, -1),  (1, 0),  (1, 1)]
+
+        # m = number of rows, n = number of columns
+        m, n = len(board), len(board[0])
+
+
+        # Helper function to count live neighbors for cell (i, j)
+        def getCount(i, j):
+            count = 0
+            for dx, dy in dirs:
+                r, c = i + dx, j + dy
+                # Check if neighbor is within grid boundaries
+                if 0 <= r < m and 0 <= c < n:
+                    # Count cells that are currently alive (1)
+                    # or were alive but are marked to die (2)
+                    if board[r][c] == 1 or board[r][c] == 2:
+                        count += 1
+            return count
+
+        # First pass: determine next state for each cell
+        # Encode transitions without disturbing neighbors:
+        #   1 -> 0 becomes 2 (was live, will die)
+        #   0 -> 1 becomes 3 (was dead, will become live)
+
+        for i in range(m):
+            for j in range(n):
+                cnt = getCount(i, j)
+
+                # Rule 4: Dead cell with exactly 3 live neighbors -> becomes live
+                if board[i][j] == 0 and cnt == 3:
+                    board[i][j] = 3
+                # Rule 1 & 3: Live cell with <2 or >3 live neighbors -> dies
+                elif board[i][j] == 1 and (cnt < 2 or cnt > 3):
+                    board[i][j] = 2
+
+
+        # Second pass: finalize the board by decoding temporary states
+
+        for i in range(m):
+            for j in range(n):
+                # 3 means dead -> live, set to 1
+                if board[i][j] == 3:
+                    board[i][j] = 1
+                # 2 means live -> dead, set to 0
+                elif board[i][j] == 2:
+                    board[i][j] = 0