LC 946. Validate Stack Sequences (Python Stack)

Author: raul-sauco

README.md

@@ -337,6 +337,7 @@ Solutions to LeetCode problems. The first column links to the problem in LeetCod
 | [936. Stamping The Sequence][lc936]                                                           | 🔴 Hard    | [![python](res/py.png)][lc936py]                                                                      |
 | [938. Range Sum of BST][lc938]                                                                | 🟢 Easy    | [![python](res/py.png)][lc938py]                                                                      |
 | [944. Delete Columns to Make Sorted][lc944]                                                   | 🟢 Easy    | [![python](res/py.png)][lc944py]                                                                      |
+| [946. Validate Stack Sequences][lc946]                                                        | 🟠 Medium  | [![python](res/py.png)][lc946py]                                                                      |
 | [947. Most Stones Removed with Same Row or Column][lc947]                                     | 🟠 Medium  | [![python](res/py.png)][lc947py]                                                                      |
 | [948. Bag of Tokens][lc948]                                                                   | 🟠 Medium  | [![python](res/py.png)][lc948py]                                                                      |
 | [953. Verifying an Alien Dictionary][lc953]                                                   | 🟢 Easy    | [![python](res/py.png)][lc953py] [![rust](res/rs.png)][lc953rs]                                       |
@@ -1172,6 +1173,8 @@ Solutions to LeetCode problems. The first column links to the problem in LeetCod
 [lc938py]: leetcode/range-sum-of-bst.py
 [lc944]: https://leetcode.com/problems/delete-columns-to-make-sorted/
 [lc944py]: leetcode/delete-columns-to-make-sorted.py
+[lc946]: https://leetcode.com/problems/validate-stack-sequences/
+[lc946py]: leetcode/validate-stack-sequences.py
 [lc947]: https://leetcode.com/problems/most-stones-removed-with-same-row-or-column/
 [lc947py]: leetcode/most-stones-removed-with-same-row-or-column.py
 [lc948]: https://leetcode.com/problems/bag-of-tokens/

leetcode/validate-stack-sequences.py

@@ -0,0 +1,65 @@
+# 946. Validate Stack Sequences
+# 🟠 Medium
+#
+# https://leetcode.com/problems/validate-stack-sequences/
+#
+# Tags: Array - Stack - Simulation
+
+import timeit
+from typing import List
+
+
+# Use an extra stack of memory and simulate the operations that took
+# place, push the next element, then, while the top of the stack matches
+# the next element that needs to be popped, pop it.
+#
+# Time complexity: O(n) - We visit each element on the pushed array and
+# do amortized O(1) for each.
+# Space complexity: O(n) - The stack can grow to the same size as the
+# input.
+#
+# Runtime 57 ms Beats 99.86%
+# Memory 14.1 MB Beats 85.31%
+class Solution:
+    def validateStackSequences(
+        self, pushed: List[int], popped: List[int]
+    ) -> bool:
+        # Try to simulate the stack, we pop when we can and push
+        # when we cannot pop.
+        n, stack, next_pop = len(pushed), [], 0
+        for el in pushed:
+            # Push the next element.
+            stack.append(el)
+            # Pop all elements that match.
+            while stack and next_pop < n and stack[-1] == popped[next_pop]:
+                stack.pop()
+                next_pop += 1
+        # If the sequence is valid, we would have popped everything.
+        return next_pop == n
+
+
+def test():
+    executors = [Solution]
+    tests = [
+        [[1, 2, 3, 4, 5], [4, 5, 3, 2, 1], True],
+        [[1, 2, 3, 4, 5], [4, 3, 5, 1, 2], False],
+    ]
+    for executor in executors:
+        start = timeit.default_timer()
+        for _ in range(1):
+            for col, t in enumerate(tests):
+                sol = executor()
+                result = sol.validateStackSequences(t[0], t[1])
+                exp = t[2]
+                assert result == exp, (
+                    f"\033[93m» {result} <> {exp}\033[91m for"
+                    + f" test {col} using \033[1m{executor.__name__}"
+                )
+        stop = timeit.default_timer()
+        used = str(round(stop - start, 5))
+        cols = "{0:20}{1:10}{2:10}"
+        res = cols.format(executor.__name__, used, "seconds")
+        print(f"\033[92m» {res}\033[0m")
+
+
+test()