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Leetcode剑指Offer刷题 - 第二十七天

花明 2022-02-19 阅读 53

剑指 Offer 59 - I. 滑动窗口的最大值 

解法一:暴力模拟

class Solution {
    public int[] maxSlidingWindow(int[] nums, int k) {
        if (nums.length == 0) return new int[0];
        int len = nums.length - k + 1;
        List<Integer> list = new ArrayList<>();
        int[] ret = new int[len];
        
        for (int i = 0; i < len; ++i) {
            int max = Integer.MIN_VALUE;
            for (int j = i; j < i + k; ++j) {
                max = Math.max(nums[j], max);
            }
            list.add(max);
        }
        for (int i = 0; i < len; ++i) {
            ret[i] = list.get(i);
        }
        return ret;
    }
}

解法二:单调队列(优化版)

class Solution {
    public int[] maxSlidingWindow(int[] nums, int k) {
        if (nums.length == 0 || k == 0) return new int[0];
        Deque<Integer> deque = new LinkedList<>();
        int[] ret = new int[nums.length - k + 1];
        int index = 0;
        //未形成窗口区间
        for (int i = 0; i < k; ++i) {
            //队列不为空时,当前值与队列尾部值比较,如果大于,删除队列尾部值
            //一直循环删除到队列中的值都大于当前值,或者删到队列为空
            while (!deque.isEmpty() && nums[i] > deque.peekLast()) {
                deque.removeLast();
            }
            //执行完上面的循环后,队列中要么为空,要么值都比当前值大,然后就把当前值添加到队列中
            deque.addLast(nums[i]);
        }

        //窗口区间刚形成后,把队列首位值添加到队列中
        ret[index++] = deque.peekFirst();

        //窗口区间形成
        for (int i = k; i < nums.length; ++i) {
            //i-k是已经在区间外了,如果首位等于nums[i-k],那么说明此时首位值已经不再区间内了,需要删除
            if (deque.peekFirst() == nums[i - k]) {
                deque.removeFirst();
            }
            //删除队列中比当前值小的值
            while (!deque.isEmpty() && deque.peekLast() < nums[i]) {
                deque.removeLast();
            }
            //把当前值添加到队列中
            deque.addLast(nums[i]);
            //把队列的首位值添加到arr数组中
            ret[index++] = deque.peekFirst();
        }

        return ret;
    }
}

剑指 Offer 59 - II. 队列的最大值

解法:滑动窗口

class MaxQueue {
    Deque<Integer> ret, max;

    public MaxQueue() {
        ret = new LinkedList<Integer>();
        max = new LinkedList<Integer>();
    }

    public int max_value() {
        if (max.isEmpty()) return -1;
        return max.peekFirst();
    }

    public void push_back(int value) {
        ret.addLast(value);
        while (!max.isEmpty() && max.peekLast() < value) max.removeLast();
        max.addLast(value);
    }

    public int pop_front() {
        if (ret.isEmpty()) return -1;
        int temp = ret.peekFirst();
        if (temp == max.peekFirst()) max.removeFirst();
        ret.removeFirst();
        return temp;
    }
}
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