Codeforces #527 D2

#include <bits/stdc++.h>
#pragma comment(linker, "/stack:200000000")
#pragma GCC optimize ("Ofast")
#pragma GCC target ("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#include <cstdio>
#include <cstring>
#include <cmath>
#include <cstring>
#include <chrono>
#include <complex>




#define endl "\n"
#define ll long long int
#define vi vector<int>
#define vll vector<ll>
#define vvi vector < vi >
#define pii pair<int,int>
#define pll pair<long long, long long>
#define vpll vector < pair<long long,long long> >
#define mod 1000000007
#define inf 1000000000000000001;
#define all(c) c.begin(),c.end()
#define fill(a)  memset(a, 0, sizeof (a))
#define mp make_pair
#define pb push_back
#define mem(a,val) memset(a,val,sizeof(a))
#define eb emplace_back
#define f first
#define s second

#define rep(i, n)    for(long long int i = 0; i < (n); ++i)
#define repA(i, a, n)  for(int i = a; i <= (n); ++i)
#define repD(i, a, n)  for(int i = a; i >= (n); --i)
#define trav(a, x) for(auto& a : x)


using namespace std;


struct StackNode
{
    ll data;
    struct StackNode* next;
};

struct StackNode* newNode(ll data)
{
    struct StackNode* stackNode =
              (struct StackNode*) malloc(sizeof(struct StackNode));
    stackNode->data = data;
    stackNode->next = NULL;
    return stackNode;
}

ll isEmpty(struct StackNode *root)
{
    return !root;
}

void push(struct StackNode** root, ll data)
{
    struct StackNode* stackNode = newNode(data);
    stackNode->next = *root;
    *root = stackNode;
    // printf("%d pushed to stack\n", data);
}

int pop(struct StackNode** root)
{
    if (isEmpty(*root))
        return INT_MIN;
    struct StackNode* temp = *root;
    *root = (*root)->next;
    ll popped = temp->data;
    free(temp);

    return popped;
}

ll peek(struct StackNode* root)
{
    if (isEmpty(root))
        return INT_MIN;
    return root->data;
}



int main(){
	std::ios::sync_with_stdio(false);
        cin.tie(0);
        cin.exceptions(cin.failbit);
	ll t,n,i,j,d,q,x,y,r,m;
	vll v;
    vpll v2;
    struct StackNode* root = NULL;
    cin>>n;
    cin>>d;
    push(&root, d);
    ll max = d;
    ll prev = 0;
    ll spl = 0;
    rep(i,n){
        if(i==0) continue;
        cin>>d;
        if((peek(root)==d)){
            prev = pop(&root);
            if(!isEmpty(root))
            if(peek(root)<prev){
                spl = 1;
                break;
            }
        }
        else push(&root,d);
    }
    ll flag = 0;
    if(isEmpty(root)) flag = 1;
    else{
        pop(&root);
        if(isEmpty(root)) flag = 1;
    }
    if(spl==1) flag = 0;
    if(flag==1) cout<<"YES";
    else cout<<"NO";
}

/*input




Check the end cases
*/