金字塔转换矩阵

/*
思路：
1. 使用流API创建一个递归函数来尝试构建金字塔的每一层。
2. 在每一层中，检查当前层的所有可能组合，并查看它们是否在允许的模式中。
3. 如果我们能够通过所有层的检查并到达顶部，则返回true。
*/
import java.util.*;
import java.util.stream.*;
 
public class PyramidTransitionStream {
    public boolean pyramidTransition(String bottom, List<String> allowed) {
        Map<String, List<Character>> allowedMap = allowed.stream().collect(Collectors.groupingBy(s -> s.substring(0, 2), Collectors.mapping(s -> s.charAt(2), Collectors.toList())));
        return canBuildPyramid(bottom, allowedMap);
    }
 
    private boolean canBuildPyramid(String bottom, Map<String, List<Character>> allowedMap) {
        if (bottom.length() == 1) return true;
        if (IntStream.range(0, bottom.length() - 1).anyMatch(i -> !allowedMap.containsKey(bottom.substring(i, i + 2)))) return false;
        List<String> nextLevel = new ArrayList<>();
        generateNextLevel(bottom, 0, new StringBuilder(), nextLevel, allowedMap);
        return nextLevel.stream().anyMatch(next -> canBuildPyramid(next, allowedMap));
    }
 
    private void generateNextLevel(String bottom, int index, StringBuilder current, List<String> nextLevel, Map<String, List<Character>> allowedMap) {
        if (index == bottom.length() - 1) {
            nextLevel.add(current.toString());
            return;
        }
        String base = bottom.substring(index, index + 2);
        allowedMap.get(base).forEach(c -> {
            current.append(c);
            generateNextLevel(bottom, index + 1, current, nextLevel, allowedMap);
            current.deleteCharAt(current.length() - 1);
        });
    }
}