给定一个由前n个自然数组成的队列(随机顺序)。任务是检查是否可以使用堆栈将给定的Queue元素按升序排列在另一个Queue中。允许的操作是：
1.从堆栈中推入和弹出元素
2.从给定的队列中弹出(或入队)。
3.在另一个队列中推送(或出队)。
例子 ：
Input : Queue[] = { 5, 1, 2, 3, 4 }
Output : Yes
Pop the first element of the given Queue i.e 5.
Push 5 into the stack.
Now, pop all the elements of the given Queue and push them to
second Queue.
Now, pop element 5 in the stack and push it to the second Queue.
Input : Queue[] = { 5, 1, 2, 6, 3, 4 }
Output : No
Push 5 to stack.
Pop 1, 2 from given Queue and push it to another Queue.
Pop 6 from given Queue and push to stack.
Pop 3, 4 from given Queue and push to second Queue.
Now, from using any of above operation, we cannot push 5
into the second Queue because it is below the 6 in the stack.
观察，第二个Queue(将包含已排序的元素)从给定的Queue或Stack中获取输入(或排队元素)。因此，下一个预期的元素(最初将是1)必须作为给定Queue的前元素或Stack的顶层元素出现。因此，只需将期望的元素初始化为1即可模拟第二个Queue的过程，并检查是否可以从给定Queue的顶部或堆栈的顶部获取期望的元素。如果我们不能从其中任何一个中获取它，则弹出给定Queue的最前面的元素，并将其推入堆栈。
另外，请注意，堆栈也必须在每个实例中都进行排序，即，堆栈顶部的元素必须在堆栈中最小。例如。令x> y，则x总是在y之前。因此，不能将x推入堆栈中的y之前。因此，我们不能将具有较高值的元素推到具有较小值的元素的顶部。
算法：
1.初始化Expected_element = 1
2.检查给定Queue的最前面的元素或堆栈的最前面的元素是否具有Expected_element
….a)如果是，则将Expected_element加1，重复步骤2。
….b)否则，弹出Queue的前面，并将其推入堆栈。如果弹出的元素大于堆栈顶部，则返回“否”。
以下是此方法的实现：
C++
// CPP Program to check if a queue of first
// n natural number can be sorted using a stack
#include
using namespace std;
// Function to check if given queue element
// can be sorted into another queue using a
// stack.
bool checkSorted(int n, queue& q)
{
stack st;
int expected = 1;
int fnt;
// while given Queue is not empty.
while (!q.empty()) {
fnt = q.front();
q.pop();
// if front element is the expected element
if (fnt == expected)
expected++;
else {
// if stack is empty, push the element
if (st.empty()) {
st.push(fnt);
}
// if top element is less than element which
// need to be pushed, then return fasle.
else if (!st.empty() && st.top() < fnt) {
return false;
}
// else push into the stack.
else
st.push(fnt);
}
// while expected element are coming from
// stack, pop them out.
while (!st.empty() && st.top() == expected) {
st.pop();
expected++;
}
}
// if the final expected element value is equal
// to initial Queue size and the stack is empty.
if (expected - 1 == n && st.empty())
return true;
return false;
}
// Driven Program
int main()
{
queue q;
q.push(5);
q.push(1);
q.push(2);
q.push(3);
q.push(4);
int n = q.size();
(checkSorted(n, q) ? (cout << "Yes") :
(cout << "No"));
return 0;
} Java
// Java Program to check if a queue
// of first n natural number can
// be sorted using a stack
import java.io.*;
import java.util.*;
class GFG
{
static Queue q =
new LinkedList();
// Function to check if given
// queue element can be sorted
// into another queue using a stack.
static boolean checkSorted(int n)
{
Stack st =
new Stack();
int expected = 1;
int fnt;
// while given Queue
// is not empty.
while (q.size() != 0)
{
fnt = q.peek();
q.poll();
// if front element is
// the expected element
if (fnt == expected)
expected++;
else
{
// if stack is empty,
// push the element
if (st.size() == 0)
{
st.push(fnt);
}
// if top element is less than
// element which need to be
// pushed, then return fasle.
else if (st.size() != 0 &&
st.peek() < fnt)
{
return false;
}
// else push into the stack.
else
st.push(fnt);
}
// while expected element are
// coming from stack, pop them out.
while (st.size() != 0 &&
st.peek() == expected)
{
st.pop();
expected++;
}
}
// if the final expected element
// value is equal to initial Queue
// size and the stack is empty.
if (expected - 1 == n &&
st.size() == 0)
return true;
return false;
}
// Driver Code
public static void main(String args[])
{
q.add(5);
q.add(1);
q.add(2);
q.add(3);
q.add(4);
int n = q.size();
if (checkSorted(n))
System.out.print("Yes");
else
System.out.print("No");
}
}
// This code is contributed by
// Manish Shaw(manishshaw1) Python3
# Python Program to check if a queue of first
# n natural number can be sorted using a stack
from queue import Queue
# Function to check if given queue element
# can be sorted into another queue using a
# stack.
def checkSorted(n, q):
st = []
expected = 1
fnt = None
# while given Queue is not empty.
while (not q.empty()):
fnt = q.queue[0]
q.get()
# if front element is the
# expected element
if (fnt == expected):
expected += 1
else:
# if stack is empty, put the element
if (len(st) == 0):
st.append(fnt)
# if top element is less than element which
# need to be puted, then return fasle.
elif (len(st) != 0 and st[-1] < fnt):
return False
# else put into the stack.
else:
st.append(fnt)
# while expected element are coming
# from stack, pop them out.
while (len(st) != 0 and
st[-1] == expected):
st.pop()
expected += 1
# if the final expected element value is equal
# to initial Queue size and the stack is empty.
if (expected - 1 == n and len(st) == 0):
return True
return False
# Driver Code
if __name__ == '__main__':
q = Queue()
q.put(5)
q.put(1)
q.put(2)
q.put(3)
q.put(4)
n = q.qsize()
if checkSorted(n, q):
print("Yes")
else:
print("No")
# This code is contributed by PranchalKC#
// C# Program to check if a queue
// of first n natural number can
// be sorted using a stack
using System;
using System.Linq;
using System.Collections.Generic;
class GFG
{
// Function to check if given
// queue element can be sorted
// into another queue using a stack.
static bool checkSorted(int n,
ref Queue q)
{
Stack st = new Stack();
int expected = 1;
int fnt;
// while given Queue
// is not empty.
while (q.Count != 0)
{
fnt = q.Peek();
q.Dequeue();
// if front element is
// the expected element
if (fnt == expected)
expected++;
else
{
// if stack is empty,
// push the element
if (st.Count != 0)
{
st.Push(fnt);
}
// if top element is less than
// element which need to be
// pushed, then return fasle.
else if (st.Count != 0 &&
st.Peek() < fnt)
{
return false;
}
// else push into the stack.
else
st.Push(fnt);
}
// while expected element are
// coming from stack, pop them out.
while (st.Count != 0 &&
st.Peek() == expected)
{
st.Pop();
expected++;
}
}
// if the final expected element
// value is equal to initial Queue
// size and the stack is empty.
if (expected - 1 == n &&
st.Count == 0)
return true;
return false;
}
// Driver Code
static void Main()
{
Queue q = new Queue();
q.Enqueue(5);
q.Enqueue(1);
q.Enqueue(2);
q.Enqueue(3);
q.Enqueue(4);
int n = q.Count;
if (checkSorted(n, ref q))
Console.Write("Yes");
else
Console.Write("No");
}
}
// This code is contributed by
// Manish Shaw(manishshaw1) 输出 ：
Yes
视频由Parul Shandilya提供