Added stack Problems(medium Leetcode) Rainwater trapping, asteroid co…

Algorithms_and_Data_Structures/Stack/AsteroidCollision.py

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+def asteroid_collision(asteroids):
+    """
+    LeetCode #735 - Asteroid Collision
+    ----------------------------------
+    Simulate asteroid collisions moving along a line.
+    Positive -> right, Negative -> left.
+    """
+    stack = []
+    for a in asteroids:
+        while stack and stack[-1] > 0 and a < 0 and stack[-1] < abs(a):
+            stack.pop()
+        if not stack or a > 0 or stack[-1] < 0:
+            stack.append(a)
+        elif stack[-1] == -a:
+            stack.pop()
+    return stack
+
+
+if __name__ == "__main__":
+    arr = list(map(int, input("Enter asteroid values separated by space: ").split()))
+    print("Result after collisions:", asteroid_collision(arr))

Algorithms_and_Data_Structures/Stack/NextGreaterElementii.py

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+def next_greater_elements(nums):
+    """
+    LeetCode #503 - Next Greater Element II
+    ---------------------------------------
+    Given a circular array, find the next greater element 
+    for each element in the array.
+    """
+    n = len(nums)
+    ans = [-1] * n
+    stack = []
+
+    for i in range(2 * n - 1, -1, -1):
+        while stack and stack[-1] <= nums[i % n]:
+            stack.pop()
+        if i < n:
+            ans[i] = stack[-1] if stack else -1
+        stack.append(nums[i % n])
+
+    return ans
+
+
+if __name__ == "__main__":
+    arr = list(map(int, input("Enter numbers separated by space: ").split()))
+    print("Next greater elements:", next_greater_elements(arr))

Algorithms_and_Data_Structures/Stack/TrappingRainWater.py

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+def trapping_rain_water(height):
+    """
+    LeetCode #42 - Trapping Rain Water
+    ----------------------------------
+    Given n non-negative integers representing an elevation map 
+    where the width of each bar is 1, compute how much water is trapped.
+    """
+    n = len(height)
+    if n == 0:
+        return 0
+
+    left_max = [0] * n
+    right_max = [0] * n
+
+    left_max[0] = height[0]
+    for i in range(1, n):
+        left_max[i] = max(height[i], left_max[i - 1])
+
+    right_max[n - 1] = height[n - 1]
+    for i in range(n - 2, -1, -1):
+        right_max[i] = max(height[i], right_max[i + 1])
+
+    total = 0
+    for i in range(n):
+        total += min(left_max[i], right_max[i]) - height[i]
+
+    return total
+
+
+if __name__ == "__main__":
+    arr = list(map(int, input("Enter heights separated by space: ").split()))
+    print("Water trapped:", trapping_rain_water(arr))