滑动窗口，单调栈模板

单调队列，滑动窗口

题目链接：https://www.acwing.com/problem/content/156/

滑动窗口，维护一个大小为k的队列，使其队列里面的值是单调的。

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;

public class Main {
    static BufferedReader bf = new BufferedReader(new InputStreamReader(System.in));
    static BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(System.out));
    static final int N = (int)1e6 + 10;
    // 队列中存下标
    static int[] q = new int[N];
    static int hh = 0, tt = -1;
    public static void main(String[] args) throws IOException {
        String[] str = bf.readLine().split(" ");
        int n = Integer.parseInt(str[0]);
        int k = Integer.parseInt(str[1]);
        int[] a = new int[n + 1];
        String[] str1 = bf.readLine().split(" ");
        for(int i = 0 ; i < n; i++) {
            a[i] = Integer.parseInt(str1[i]); // 输入
            // 当下标 离 i的距离大于 k时，出队
            if(i - k + 1 > q[hh])hh++;
            // 当a[i] 小于 a[q[tt]] 时， 出队（维护最小的在队首）
            while(hh <= tt && a[i] <= a[q[tt]])tt--;
            // 入队
            q[++tt] = i;
            if(i + 1 >= k)bw.write(a[q[hh]] + " ");
        }
        bw.write("\n");
        hh = 0; tt = -1;
        for(int i = 0 ; i < n; i++) {
            a[i] = Integer.parseInt(str1[i]);
            if(i - k + 1 > q[hh])hh++;
            // 反之 依然
            while(hh <= tt && a[i] >= a[q[tt]])tt--;

            q[++tt] = i;
            if(i + 1 >= k)bw.write(a[q[hh]] + " ");
        }
        bw.write("\n");
        bw.flush();
    }

}

单调栈

栈内元素单调；

题目链接：https://www.acwing.com/problem/content/832/

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.util.Stack;


public class Main {
    static BufferedReader bf = new BufferedReader(new InputStreamReader(System.in));
    static BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(System.out));
    public static void main(String[] args) throws Exception {
        int n = Integer.valueOf(bf.readLine());
        String[] str = bf.readLine().split(" ");
        int[] a = new int[n];

        Stack<Integer> stack = new Stack<Integer>();
        for(int i = 0; i < n; i++) a[i] = Integer.valueOf(str[i]);

        for(int i = 0; i < n; i++) {
            //			int ans = -1; 如果栈内 从底 到 Top是 从大到小的，保证每次peek都是最大的
            while(!stack.isEmpty() && stack.peek() >= a[i]) {
                stack.pop();
            }
            if(stack.isEmpty())bw.write("-1 ");
            else bw.write(Integer.valueOf(stack.peek()).toString() + " ");
            stack.push(a[i]);
        }
        bw.flush();

    }

}