0837. New 21 Game

https://leetcode.com/problems/new-21-game

Description

Alice plays the following game, loosely based on the card game "21".

Alice starts with 0 points and draws numbers while she has less than k points. During each draw, she gains an integer number of points randomly from the range [1, maxPts], where maxPts is an integer. Each draw is independent and the outcomes have equal probabilities.

Alice stops drawing numbers when she gets k or more points.

Return the probability that Alice has n or fewer points.

Answers within 10-5 of the actual answer are considered accepted.

Example 1:

**Input:** n = 10, k = 1, maxPts = 10
**Output:** 1.00000
**Explanation:** Alice gets a single card, then stops.

Example 2:

**Input:** n = 6, k = 1, maxPts = 10
**Output:** 0.60000
**Explanation:** Alice gets a single card, then stops.
In 6 out of 10 possibilities, she is at or below 6 points.

Example 3:

**Input:** n = 21, k = 17, maxPts = 10
**Output:** 0.73278

Constraints:

• 0 <= k <= n <= 104

• 1 <= maxPts <= 104

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