LightOJ 1050 - Marbles （概率dp）

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题意:

r<=500,b<=500个球,每回合我随机抽一个球,另一个人拿一个b球
如果他不能拿为他输,如果最后一个拿走是b球我赢,求我赢的概率

分析:

dp[r][b]:=当前状态我赢的概率,转移就我拿r球或者b球,很简单,注意边界情况

代码:

////  Created by TaoSama on 2015-11-20//  Copyright (c) 2015 TaoSama. All rights reserved.////#pragma comment(linker, "/STACK:1024000000,1024000000")#include <algorithm>#include <cctype>#include <cmath>#include <cstdio>#include <cstdlib>#include <cstring>#include <iomanip>#include <iostream>#include <map>#include <queue>#include <string>#include <set>#include <vector>using namespace std;#define pr(x) cout << #x << " = " << x << "  "#define prln(x) cout << #x << " = " << x << endlconst int N = 1e5 + 10, INF = 0x3f3f3f3f, MOD = 1e9 + 7;int r, b;double dp[505][505];double dfs(int r, int b) {    double &ret = dp[r][b];    if(ret >= 0) return ret;    if(r == 0) return ret = 1;    if(b == 0) return ret = 0;    int sum = r + b;    ret = 1.0 * r / sum * dfs(r - 1, b - 1); //I take red    if(b > 1) ret += 1.0 * b / sum * dfs(r, b - 2); //I take blue    return ret;}int main() {#ifdef LOCAL    freopen("C:\\Users\\TaoSama\\Desktop\\in.txt", "r", stdin);//  freopen("C:\\Users\\TaoSama\\Desktop\\out.txt","w",stdout);#endif    ios_base::sync_with_stdio(0);    int t; scanf("%d", &t);    int kase = 0;    fill(dp[0], dp[0] + 505 * 505, -1.0);    while(t--) {        scanf("%d%d", &r, &b);        printf("Case %d: %.10f\n", ++kase, dfs(r, b));    }    return 0;}

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