26. Validate Stack Sequences.cpp

/*
Validate Stack Sequences
========================

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.
*/

class Solution
{
public:
  bool validateStackSequences(vector<int> &pushed, vector<int> &popped)
  {
    stack<int> s;
    int i = 0, j = 0, n = pushed.size();

    while (i < n)
    {
      s.push(pushed[i]);
      i++;

      while (s.size() && j >= 0 && j < n && s.top() == popped[j])
      {
        s.pop();
        j++;
      }
    }

    while (s.size() && j >= 0 && j < n && s.top() == popped[j])
    {
      s.pop();
      j++;
    }

    return s.size() == 0;
  }
};