There is a restaurant with a single chef. You are given an array customers, where customers[i] = [arrivali, timei]:

arrivaliis the arrival time of the ithcustomer. The arrival times are sorted in non-decreasing order.

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timeiis the time needed to prepare the order of the ithcustomer.

When a customer arrives, he gives the chef his order, and the chef starts preparing it once he is idle. The customer waits till the chef finishes preparing his order. The chef does not prepare food for more than one customer at a time. The chef prepares food for customers in the order they were given in the input.

Return the average waiting time of all customers. Solutions within 10-5from the actual answer are considered accepted.

Example 1:

Input: customers = [[1,2],[2,5],[4,3]]

Output: 5.00000

Explanation:

1) The first customer arrives at time 1, the chef takes his order and starts preparing it immediately at time 1, and finishes at time 3, so the waiting time of the first customer is 3 - 1 = 2. 

2) The second customer arrives at time 2, the chef takes his order and starts preparing it at time 3, and finishes at time 8, so the waiting time of the second customer is 8 - 2 = 6. 

3) The third customer arrives at time 4, the chef takes his order and starts preparing it at time 8, and finishes at time 11, so the waiting time of the third customer is 11 - 4 = 7.So the average waiting time = (2 + 6 + 7) / 3 = 5.

Example 2:

Input: customers = [[5,2],[5,4],[10,3],[20,1]]Output: 3.25000

Explanation:

1) The first customer arrives at time 5, the chef takes his order and starts preparing it immediately at time 5, and finishes at time 7, so the waiting time of the first customer is 7 - 5 = 2. 

2) The second customer arrives at time 5, the chef takes his order and starts preparing it at time 7, and finishes at time 11, so the waiting time of the second customer is 11 - 5 = 6. 

3) The third customer arrives at time 10, the chef takes his order and starts preparing it at time 11, and finishes at time 14, so the waiting time of the third customer is 14 - 10 = 4. 

4) The fourth customer arrives at time 20, the chef takes his order and starts preparing it immediately at time 20, and finishes at time 21, so the waiting time of the fourth customer is 21 - 20 = 1.So the average waiting time = (2 + 6 + 4 + 1) / 4 = 3.25.

Constraints:

1 <= customers.length <= 105

1 <= arrivali, timei <= 104

arrivali <= arrivali+1

其实就是放两个变量，1个变量是每次等待的时间，另一个变量是每次做饭完成的时间点，

每次去判断完成的时间点跟客人到达的时间去比较；

完成的时间超过客人到达的时间，那么就要多等一会了，多等的时间是完成的时间减去客户到达的时间。

如果完成的时间小于客人到达的时间，那么就是厨师多等一会了，客人多等的时间就是厨师做饭的时间了。

下面是代码：

Runtime: 3 ms, faster than 75.19% of Java online submissions for Average Waiting Time.

Memory Usage: 87.1 MB, less than 94.74% of Java online submissions for Average Waiting Time.