946. Validate Stack Sequences

Description

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.

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.

Approach

Links

• GeeksforGeeks

• Leetcode

• ProgramCreek

• YouTube

Examples

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.

Solutions

Solution 1

/**
 * Time complexity : O(N), where N is the length of pushed and popped.
 * Space complexity : O(N)
 */

class Solution {
    public boolean validateStackSequences(int[] pushed, int[] popped) {
        int N = pushed.length;
        Stack<Integer> stack = new Stack();

        int j = 0;
        for (int x: pushed) {
            stack.push(x);
            while (!stack.isEmpty() && j < N && stack.peek() == popped[j]) {
                stack.pop();
                j++;
            }
        }

        return j == N;
    }
}

Solution 2

/**
 * Time complexity : O(N), where N is the length of pushed and popped.
 * Space complexity : O(1)
 */
 
class Solution {
    public boolean validateStackSequences(int[] pushed, int[] popped) {
        int n = pushed.length, top = 0;
        
		for(int pushIdx = 0, popIdx = 0; pushIdx < n; pushIdx++) {
            // Push kth element to stack
            pushed[top] = pushed[pushIdx];
            
            // Pop if element match
            while(top >= 0 && pushed[top] == popped[popIdx]) {
                top--;
                popIdx++;
            }
            
            top++;
        }
        return top == 0;
    }
}

Follow up