src/main/java/com/fishercoder/solutions/_756.java

package com.fishercoder.solutions;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

/**
 * 756. Pyramid Transition Matrix
 *
 * We are stacking blocks to form a pyramid. Each block has a color which is a one letter string, like `'Z'`.
 * For every block of color `C` we place not in the bottom row,
 * we are placing it on top of a left block of color `A` and right block of color `B`.
 * We are allowed to place the block there only if `(A, B, C)` is an allowed triple.
 * We start with a bottom row of bottom,
 * represented as a single string. We also start with a list of allowed triples allowed.
 * Each allowed triple is represented as a string of length 3.
 * Return true if we can build the pyramid all the way to the top, otherwise false.

 Example 1:
 Input: bottom = "XYZ", allowed = ["XYD", "YZE", "DEA", "FFF"]
 Output: true
 Explanation:
 We can stack the pyramid like this:
     A
    / \
   D   E
  / \ / \
 X   Y   Z

 This works because ('X', 'Y', 'D'), ('Y', 'Z', 'E'), and ('D', 'E', 'A') are allowed triples.

 Example 2:
 Input: bottom = "XXYX", allowed = ["XXX", "XXY", "XYX", "XYY", "YXZ"]
 Output: false
 Explanation:
 We can't stack the pyramid to the top.
 Note that there could be allowed triples (A, B, C) and (A, B, D) with C != D.

 Note:
 bottom will be a string with length in range [2, 8].
 allowed will have length in range [0, 200].
 Letters in all strings will be chosen from the set {'A', 'B', 'C', 'D', 'E', 'F', 'G'}.
 */
public class _756 {
  public static class Solution1 {
    /**credit: https://discuss.leetcode.com/topic/116042/java-solution-map-backtracking*/
    public boolean pyramidTransition(String bottom, List<String> allowed) {
      Map<String, List<String>> map = new HashMap<>();
      for (String s : allowed) {
        String key = s.substring(0, 2);
        if (!map.containsKey(key)) {
          map.put(key, new ArrayList<>());
        }
        map.get(key).add(s.substring(2));
      }

      return helper(bottom, map);
    }

    private boolean helper(String bottom, Map<String, List<String>> map) {
      if (bottom.length() == 1) {
        return true;
      }
      for (int i = 0; i < bottom.length() - 1; i++) {
        if (!map.containsKey(bottom.substring(i, i + 2))) {
          return false;
        }
      }
      List<String> ls = new ArrayList<>();
      getList(bottom, 0, new StringBuilder(), ls, map);
      for (String s : ls) {
        if (helper(s, map)) {
          return true;
        }
      }
      return false;
    }

    private void getList(String bottom, int idx, StringBuilder sb, List<String> ls,
        Map<String, List<String>> map) {
      if (idx == bottom.length() - 1) {
        ls.add(sb.toString());
        return;
      }
      for (String s : map.get(bottom.substring(idx, idx + 2))) {
        sb.append(s);
        getList(bottom, idx + 1, sb, ls, map);
        sb.deleteCharAt(sb.length() - 1);
      }
    }
  }
}