946 Validate Stack Sequences.py

Author: rajeevranjancom

946 Validate Stack Sequences.py

@@ -0,0 +1,67 @@
+#!/usr/bin/python3
+"""
+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.
+
+
+Note:
+
+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:
+        """
+        maintain a stack and iterate through pushed
+        """
+        j = 0
+        n = len(pushed)
+        stk = []
+        for i in range(n):
+            stk.append(pushed[i])
+            while j < n and stk and stk[-1] == popped[j]:
+                stk.pop()
+                j += 1
+
+        return j == n
+
+    def validateStackSequences(self, pushed: List[int], popped: List[int]) -> bool:
+        """
+        maintain a stack
+        """
+        i = 0
+        j = 0
+        stk = []
+        n = len(pushed)
+        while i < n and j < n:
+            while i < n and (not stk or stk[-1] != popped[j]):
+                stk.append(pushed[i])
+                i += 1
+
+            stk.pop()
+            j += 1
+
+        while j < n and stk and stk[-1] == popped[j]:
+            stk.pop()
+            j += 1
+
+        return not stk