LeetCode 837. New 21 Game
原题链接在这里:https://leetcode.com/problems/new-21-game/
题目:
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, W]
, where W
is an integer. Each draw is independent and the outcomes have equal probabilities.
Alice stops drawing numbers when she gets K
or more points. What is the probability that she has N
or less points?
Example 1:
Input: N = 10, K = 1, W = 10 Output: 1.00000 Explanation: Alice gets a single card, then stops.
Example 2:
Input: N = 6, K = 1, W = 10 Output: 0.60000 Explanation: Alice gets a single card, then stops. In 6 out of W = 10 possibilities, she is at or below N = 6 points.
Example 3:
Input: N = 21, K = 17, W = 10 Output: 0.73278
Note:
0 <= K <= N <= 10000
1 <= W <= 10000
- Answers will be accepted as correct if they are within
10^-5
of the correct answer. - The judging time limit has been reduced for this question.
题解:
When the draws sum up to K, it stops, calculate the possibility K<=sum<=N.
Think about one step earlier, sum = K-1, game is not ended and draw largest card W. K-1+W is the maximum sum could get when game is ended. If it is <= N, then for sure the possiblity when games end ans sum <= N is 1.
Because the maximum is still <= 1.
Otherwise calculate the possibility sum between K and N.
Let dp[i] denotes the possibility of that when game ends sum up to i.
i is a number could be got equally from i - m and draws value m card.
Then dp[i] should be sum of dp[i-W] + dp[i-W+1] + ... + dp[i-1], devided by W.
We only need to care about previous W value sum, accumlate winSum, reduce the possibility out of range.
Time Complexity: O(N).
Space: O(N).
AC Java:
1 class Solution { 2 public double new21Game(int N, int K, int W) { 3 if(K == 0 || K-1+W <= N){ 4 return 1; 5 } 6 7 if(K > N){ 8 return 0; 9 } 10 11 double [] dp = new double[N+1]; 12 dp[0] = 1.0; 13 double winSum = 1; 14 15 double res = 0.0; 16 for(int i = 1; i<=N; i++){ 17 dp[i] = winSum/W; 18 19 if(i<K){ 20 winSum += dp[i]; 21 }else{ 22 res += dp[i]; 23 } 24 25 if(i >= W){ 26 winSum -= dp[i-W]; 27 } 28 } 29 30 return res; 31 } 32 }