946. Validate Stack Sequences - Explanation

class Solution:
    def validateStackSequences(self, pushed: List[int], popped: List[int]) -> bool:
        i = 0
        stack = []
        for n in pushed:
            stack.append(n)
            while i < len(popped) and stack and popped[i] == stack[-1]:
                stack.pop()
                i += 1
        return not stack

class Solution:
    def validateStackSequences(self, pushed: List[int], popped: List[int]) -> bool:
        l = r = 0
        for num in pushed:
            pushed[l] = num
            l += 1
            while l > 0 and pushed[l - 1] == popped[r]:
                r += 1
                l -= 1
        return l == 0