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題目

Write a class RecentCounter to count recent requests.

It has only one method: ping(int t), where t represents some time in milliseconds.

Return the number of pings that have been made from 3000 milliseconds ago until now.

Any ping with time in [t - 3000, t] will count, including the current ping.

It is guaranteed that every call to ping uses a strictly larger value of t than before.

Example 1:

Input: inputs = ["RecentCounter","ping","ping","ping","ping"], inputs = [[],[1],[100],[3001],[3002]]
Output: [null,1,2,3,3]

Note:

1. Each test case will have at most 10000 calls to ping.
2. Each test case will call ping with strictly increasing values of t.
3. Each call to ping will have 1 <= t <= 10^9.

解題思路

當將 t 作為參數調用 ping 方法時，需要去除在 t - 3000 至 t 范圍外的值，由于每次調用 ping 的數值是從小至大，所以 t 是現在通過 ping 存儲過的最大值，不滿足條件的數據肯定只能小于 t - 3000，那從被存儲的最小值開始找直到數據滿足條件為止，返回存儲的數據量即可。

這個操作使先調用 ping 方法存儲起來的值被先刪除掉，滿足先進先出的特點，使用隊列比較合適，我也嘗試使用數組的方式，利用二分查找查找最后一位小于 t - 3000 數據元素的方式解決，不過通過 leetcode 測試用例的耗時和使用隊例的差距并不大，應該是每次滿足刪除條件的數據量不多，所以逐個遍歷和使用二分查找需要比較數據個數差距不大。

代碼實現

private Queue<Integer> queue = new LinkedList<>();
// by queue
public int ping(int t) {
    queue.add(t);
    while(queue.peek() < t - 3000) {
        queue.poll();
    }
    return queue.size();
}

private List<Integer> array = new ArrayList<>();
private int startIndex = 0;
// by array
public int pingByArray(int t) {
    array.add(t);
    int targetIndex = searchLastLess(array, t - 3000);
    if(targetIndex >= 0) {
        startIndex = targetIndex + 1;
    }

    return  array.size() - startIndex;
}

private int searchLastLess(List<Integer> array, int target) {
    if(array == null || array.size() < 1) {
        return -1;
    }

    int low = 0, high = array.size() - 1;
    while(low <= high) {
        int mid = low + ((high - low) >> 1);
        if(array.get(mid) >= target) {
            high = mid - 1;
        } else {
            if(mid == array.size() - 1 || array.get(mid + 1) >= target) {
                return mid;
            } else {
                low = mid + 1;
            }
        }
    }
    return -1;
}

代碼詳情可點擊查看 我的 GitHub 倉庫