[剑指offer][Java]滑动窗口的最大值

import java.util.ArrayList;
import java.util.ArrayDeque;
public class Solution {
    public ArrayList<Integer> maxInWindows(int [] num, int size)
    {
        ArrayList<Integer> result = new ArrayList<>();
        if (size==0) return result;
        int begin;// 滑动窗口最左边数的index
        ArrayDeque<Integer> q = new ArrayDeque<>();
        for(int i=0;i<num.length;i++){
            begin = i-size+1;
            if(q.isEmpty())
                q.add(i);
            else if(begin > q.peekFirst())// 队列最左边值已经过期，删除
                q.pollFirst();

            while((!q.isEmpty()) && num[q.peekLast()] <= num[i])
                q.pollLast();// 从队尾开始比较，把所有比他小的值丢掉
            q.add(i);

            if(begin>=0){
                result.add(num[q.peekFirst()]);// 永远是队列最左边最大
            }
        }
        return result;
    }
}