1518. Water Bottles

Given numBottles full water bottles, you can exchange numExchange empty water bottles for one full water bottle.

The operation of drinking a full water bottle turns it into an empty bottle.

Return the maximum number of water bottles you can drink.

Example 1:

Input: numBottles = 9, numExchange = 3
Output: 13
Explanation: You can exchange 3 empty bottles to get 1 full water bottle.
Number of water bottles you can drink: 9 + 3 + 1 = 13.

Example 2:

Input: numBottles = 15, numExchange = 4
Output: 19
Explanation: You can exchange 4 empty bottles to get 1 full water bottle. 
Number of water bottles you can drink: 15 + 3 + 1 = 19.

Example 3:

Input: numBottles = 5, numExchange = 5
Output: 6

Example 4:

Input: numBottles = 2, numExchange = 3
Output: 2

Constraints:

• 1 <= numBottles <= 100
• 2 <= numExchange <= 100

class Solution {
    private int res = 0;
    public int numWaterBottles(int numBottles, int numExchange) {
        int empty = 0;
        int lastempty = 0;
        while(numBottles != 0) {
            res += numBottles;
            empty = numBottles + lastempty;
            numBottles = empty / numExchange;
            lastempty = empty % numExchange;
        }
        return res;
    }
}

Three parts, numBottles is current drink, empty is all empty bottles include empty from last step, then update numBottles and lastempty.