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Formatted question description: https://leetcode.ca/all/946.html

# 946. Validate Stack Sequences (Medium)

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.


Note:

1. 0 <= pushed.length == popped.length <= 1000
2. 0 <= pushed[i], popped[i] < 1000
3. pushed is a permutation of popped.
4. pushed and popped have distinct values.

Related Topics:
Stack

## Solution 1.

• class Solution {
public boolean validateStackSequences(int[] pushed, int[] popped) {
Stack<Integer> stack = new Stack<Integer>();
int length = pushed.length;
int pushIndex = 0, popIndex = 0;
while (pushIndex < length && popIndex < length) {
stack.push(pushed[pushIndex]);
pushIndex++;
while (!stack.isEmpty() && stack.peek() == popped[popIndex]) {
stack.pop();
popIndex++;
}
}
return stack.isEmpty();
}
}

############

class Solution {
public boolean validateStackSequences(int[] pushed, int[] popped) {
Deque<Integer> stk = new ArrayDeque<>();
int j = 0;
for (int v : pushed) {
stk.push(v);
while (!stk.isEmpty() && stk.peek() == popped[j]) {
stk.pop();
++j;
}
}
return j == pushed.length;
}
}

• // OJ: https://leetcode.com/problems/validate-stack-sequences/
// Time: O(N)
// Space: O(N)
class Solution {
public:
bool validateStackSequences(vector<int>& pushed, vector<int>& popped) {
stack<int> s;
int p = 0;
for (int i : pushed) {
s.push(i);
while (s.size() && s.top() == popped[p]) {
s.pop();
++p;
}
}
return s.empty();
}
};

• class Solution:
def validateStackSequences(self, pushed: List[int], popped: List[int]) -> bool:
j, stk = 0, []
for v in pushed:
stk.append(v)
while stk and stk[-1] == popped[j]:
stk.pop()
j += 1
return j == len(pushed)

############

class Solution(object):
def validateStackSequences(self, pushed, popped):
"""
:type pushed: List[int]
:type popped: List[int]
:rtype: bool
"""
stack = []
N = len(pushed)
pi = 0
for i in range(N):
if stack and popped[i] == stack[-1]:
stack.pop()
else:
while pi < N and pushed[pi] != popped[i]:
stack.append(pushed[pi])
pi += 1
pi += 1
return not stack

• func validateStackSequences(pushed []int, popped []int) bool {
stk := []int{}
j := 0
for _, v := range pushed {
stk = append(stk, v)
for len(stk) > 0 && stk[len(stk)-1] == popped[j] {
stk = stk[:len(stk)-1]
j++
}
}
return j == len(pushed)
}

• function validateStackSequences(pushed: number[], popped: number[]): boolean {
const stk = [];
let j = 0;
for (const v of pushed) {
stk.push(v);
while (stk.length && stk[stk.length - 1] == popped[j]) {
stk.pop();
++j;
}
}
return j == pushed.length;
}


• /**
* @param {number[]} pushed
* @param {number[]} popped
* @return {boolean}
*/
var validateStackSequences = function (pushed, popped) {
let stk = [];
let j = 0;
for (const v of pushed) {
stk.push(v);
while (stk.length && stk[stk.length - 1] == popped[j]) {
stk.pop();
++j;
}
}
return j == pushed.length;
};


• public class Solution {
public bool ValidateStackSequences(int[] pushed, int[] popped) {
Stack<int> stk = new Stack<int>();
int j = 0;
foreach (int x in pushed)
{
stk.Push(x);
while (stk.Count != 0 && stk.Peek() == popped[j]) {
stk.Pop();
++j;
}
}
return stk.Count == 0;
}
}

• impl Solution {
pub fn validate_stack_sequences(pushed: Vec<i32>, popped: Vec<i32>) -> bool {
let mut stack = Vec::new();
let mut i = 0;
for &num in pushed.iter() {
stack.push(num);
while !stack.is_empty() && *stack.last().unwrap() == popped[i] {
stack.pop();
i += 1;
}
}
stack.len() == 0
}
}