单调栈

https://www.acwing.com/problem/content/832/

#include <iostream>
using namespace std;
const int N = 1e5 + 10;
int n;
int stk[N], tt;
int main()
{
    cin >> n;
    while (n--)
    {
        int x;
        cin >> x;
        while (tt && stk[tt] >= x) tt--;
        if (tt) cout << stk[tt] << ' ';
        else cout << "-1" << ' ';
        stk[++tt] = x;
    }
    puts("");
    return 0;
}