# Leetcode 946. Validate Stack Sequences

###### tags: `Leetcode`

https://leetcode.com/problems/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.

**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.

## Solution in C

Time Complexity: $O(n)$
Space Complexity: $O(n)$

bool validateStackSequences(int* pushed, int pushedSize, int* popped, int poppedSize){
    if (pushedSize<=0) return true;
    
    int q[pushedSize];
    int i=-1, j=0, k=0; //i for q, j for pushed, k for popped
    
    while(true){
        if(i!=-1 && k<poppedSize && q[i]==popped[k]){ //pop
            i--;
            k++;
        }
        else if(j<pushedSize){ //push
            i++;
            q[i]=pushed[j];
            j++;
        }
        else
            break;
    }
    return i==-1;
}