# LC 3169. Count Days Without Meetings

### [Problem link](https://leetcode.com/problems/count-days-without-meetings/)

###### tags: `leedcode` `medium` `c++`

You are given a positive integer <code>days</code> representing the total number of days an employee is available for work (starting from day 1). You are also given a 2D array <code>meetings</code> of size <code>n</code> where, <code>meetings[i] = [start_i, end_i]</code> represents the starting and ending days of meeting <code>i</code> (inclusive).

Return the count of days when the employee is available for work but no meetings are scheduled.

**Note:** The meetings may overlap.

**Example 1:**
Input: days = 10, meetings = [[5,7],[1,3],[9,10]]

Output: 2

Explanation:

There is no meeting scheduled on the 4^th and 8^th days.

**Example 2:**
Input: days = 5, meetings = [[2,4],[1,3]]

Output: 1

Explanation:

There is no meeting scheduled on the 5^thday.

**Example 3:**
Input: days = 6, meetings = [[1,6]]

Output: 0

Explanation:

Meetings are scheduled for all working days.

**Constraints:**

- 1 <= days <= 10⁹
- 1 <= meetings.length <= 10⁵
- meetings[i].length == 2
- 1 <= meetings[i][0] <= meetings[i][1] <= days

## Solution 1
#### C++

class Solution {
public:
    int countDays(int days, vector<vector<int>>& meetings) {
        sort(meetings.begin(), meetings.end());
        int meetingDay = 0;
        int left = meetings[0][0];
        int right = meetings[0][1];
        for (int i = 1; i < meetings.size(); i++) {
            if (meetings[i][0] <= right) {
                right = max(right, meetings[i][1]);
            } else {
                meetingDay += right - left + 1;
                left = meetings[i][0];
                right = meetings[i][1];
            }
        }
        meetingDay += right - left + 1;
        return days - meetingDay;
    }
};

>### Complexity
>n = meetings.length
>| | Time Complexity | Space Complexity |
>| ----------- | --------------- | ---------------- |
>| Solution 1 | O(nlogn) | O(1) |

## Note
跟[LC 56. Merge Intervals](https://hackmd.io/@Alone0506/SJyRonX43)類似
合併區間的時候計算meetingDay有幾天, 最後結果為days - meetingDay;