LeetCode-in-Java
837. New 21 Game
Medium
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 <= 1041 <= maxPts <= 104
Solution
public class Solution {
public double new21Game(int n, int k, int w) {
if (n >= k + w || k == 0) {
return 1.0;
}
double[] dp = new double[n + 1];
dp[0] = 1.0;
double res = 0.0;
double runningSum = dp[0];
for (int i = 1; i <= n; i++) {
dp[i] = runningSum / w;
if (i < k) {
runningSum += dp[i];
} else {
res += dp[i];
}
if (i - w >= 0) {
runningSum -= dp[i - w];
}
}
return res;
}
}