[LC] 946. Validate Stack Sequences

Posted 2020-12-08 xuanlu

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.

Time: O(N)

class Solution {
    public boolean validateStackSequences(int[] pushed, int[] popped) {
        LinkedList<Integer> stack = new LinkedList<>();
        int index = 0;
        for (int num : pushed) {
            stack.offerFirst(num);
            while (!stack.isEmpty() && stack.peekFirst() == popped[index]) {
                stack.pollFirst();
                index += 1;
            }
        }
        return stack.isEmpty();
    }
}