946. Validate Stack Sequence

Leetcode: 946. Validate Stack Sequences

Difficulty: Medium

Related Topics: Array, Stack, Simulation

Given two integer arrays pushed and popped each with distinct values, return true if this could have been the result of a sequence of push and pop operations on an initially empty stack, or false otherwise.

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:

• 1 <= pushed.length <= 1000
• 0 <= pushed[i] <= 1000
• All the elements of pushed are unique.
• popped.length == pushed.length
• popped is a permutation of pushed.

Solution

Simple simulation problem.

class Solution {
public:
    bool validateStackSequences(vector<int>& pushed, vector<int>& popped) {
        int idx = 0, n = popped.size();
        stack<int> stk;
        for (int x : pushed)
        {
            stk.push(x);
            while (idx < n && !stk.empty() && stk.top() == popped[idx])
                stk.pop(), ++idx;
        }
        return n == idx && stk.empty();
    }
};