统计所有可行路径

/*
 * 题目思路：
 * 使用 Java Stream 的方法来解决问题。我们可以利用 Stream API 来简化动态规划的过程。
 * 我们需要将动态规划转换为一个流处理过程，通过多次 map 和 reduce 操作来更新和计算路径数。
 */

import java.util.stream.IntStream;

public class SolutionStream {
    private static final int MOD = 1000000007;
    
    public int countRoutes(int[] locations, int start, int finish, int fuel) {
        int n = locations.length;
        int[][] dp = new int[n][fuel + 1];
        dp[start][fuel] = 1;
        
        IntStream.rangeClosed(0, fuel).map(remainingFuel -> fuel - remainingFuel).forEach(remainingFuel -> {
            IntStream.range(0, n).filter(currentCity -> dp[currentCity][remainingFuel] > 0).forEach(currentCity -> {
                IntStream.range(0, n).filter(nextCity -> currentCity != nextCity).forEach(nextCity -> {
                    int cost = Math.abs(locations[currentCity] - locations[nextCity]);
                    if (remainingFuel >= cost) {
                        dp[nextCity][remainingFuel - cost] = (dp[nextCity][remainingFuel - cost] + dp[currentCity][remainingFuel]) % MOD;
                    }
                });
            });
        });
        
        return IntStream.rangeClosed(0, fuel).map(remainingFuel -> dp[finish][remainingFuel]).reduce(0, (a, b) -> (a + b) % MOD);
    }
}