Add Py solution to LC946

Author: kate-melnykova

LC946-Validate-Stack-Sequences.py

@@ -0,0 +1,69 @@
+"""
+Given two sequences pushed and popped with distinct values, return true if
+and only if this could have been the result of a sequence of push and pop
+operations on an initially empty stack.
+
+
+Example 1:
+Input: pushed = [1,2,3,4,5], popped = [4,5,3,2,1]
+Output: true
+Explanation: We might do the following sequence:
+push(1), push(2), push(3), push(4), pop() -> 4,
+push(5), pop() -> 5, pop() -> 3, pop() -> 2, pop() -> 1
+
+Example 2:
+Input: pushed = [1,2,3,4,5], popped = [4,3,5,1,2]
+Output: false
+Explanation: 1 cannot be popped before 2.
+
+
+Constraints:
+(*) 0 <= pushed.length == popped.length <= 1000
+(*) 0 <= pushed[i], popped[i] < 1000
+(*) pushed is a permutation of popped.
+(*) pushed and popped have distinct values.
+"""
+from typing import List
+
+
+class Solution:
+    def validateStackSequences(self, pushed: List[int], popped: List[int]) -> bool:
+        """
+        Runtime complexity: O(n)
+        Space complexity: O(n)
+        """
+        i_popped = 0
+        i_pushed = 0
+        stack = list()
+        while i_pushed < len(pushed):
+            # we may keep pushing
+            # check if popping is possible
+            if stack and i_popped < len(popped) and stack[-1] == popped[i_popped]:
+                stack.pop()
+                i_popped += 1
+            else:
+                stack.append(pushed[i_pushed])
+                i_pushed += 1
+
+        while i_popped < len(popped):
+            if not stack or stack[-1] != popped[i_popped]:
+                return False
+
+            stack.pop()
+            i_popped += 1
+        return True
+
+
+if __name__ == '__main__':
+    from run_tests import run_tests
+
+    correct_answers = [
+        [[1,2,3,4,5], [4,5,3,2,1], True],
+        [[1,2,3,4,5], [4,3,5,1,2], False],
+        [[1,2,3,4,5], [4,3,5,2,1], True],
+        [[1,2,3,4,5], [4,5,3,1,2], False]
+    ]
+    methods = ['validateStackSequences', ]
+    for method in methods:
+        print(f'Running tests for {method}')
+        run_tests(getattr(Solution(), method), correct_answers)