P1886 AC但有个地方有点不太懂（求助）

#include<iostream>
#include<cstring>
#include<string>
#include<algorithm>
#include<cmath>
#include<bits/stdc++.h>

using namespace std;

#define itn int
#define ll long long
#define ull unsigned long long
#define ff()  for(int i=1;i<=n;i++)

const int maxn=91451451;

deque<int> q;

int n , m , num;
int a[maxn];
int b[maxn];

inline int read(){//神奇的读优
    int h_=1,d_=0;char t_=getchar();
    while(t_<'0' || t_>'9'){if(t_=='-')h_=-1;t_=getchar();}
    while(t_>='0' && t_<='9'){d_=d_*10+t_-'0';t_=getchar();}return h_*d_;
}
void print(int x_){//快输
    if(x_<0){
        putchar('-'),x_=-x_;
    }
    if(x_>9){
        print(x_/10);
    }
    putchar(x_%10+'0');
    return ;
}

signed main(){
    n = read();
    m = read();
    ff()
    {
        cin >> a[i];
    }
    for (int i = 1;i <= n; i++)//min
    {
        while (!q.empty() && a[q.back()] > a[i])
        {
            q.pop_back();
        }
        q.push_back(i);
        if (i >= m )
        {
            while(!q.empty()&&q.front()<=i-m) q.pop_front();
            cout<<a[q.front()]<< ' ';
        }

    }
    cout<<'\n';
    while(!q.empty())q.pop_front();
    for (int i = 1;i <= n; i++)//max
    {
        while (!q.empty() && a[q.back()] < a[i])
        {
            q.pop_back();
        }
        q.push_back(i);
        if (i >= m )
        {
            while(!q.empty()&&q.front()<=i-m) q.pop_front();
            cout<<a[q.front()]<<' ';
        }

    }
    cout<<'\n';
    return 0;
}

那几个while循环都有点懵逼，请大佬解答一下是什么个原理

万分感谢~~~