LC 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:

1. bottom will be a string with length in range [2, 8].
2. allowed will have length in range [0, 200].
3. Letters in all strings will be chosen from the set {'A', 'B', 'C', 'D', 'E', 'F', 'G'}.

map + backtracing

Runtime: 9 ms, faster than 80.22% of Java online submissions for Pyramid Transition Matrix.

class Solution {
  private Map<String, List<Character>> mp = new HashMap<>();
  public boolean pyramidTransition(String bottom, List<String> allowed) {
    for(int i=0; i<allowed.size(); i++){
      String tmp = allowed.get(i).substring(0,2);
      if(mp.containsKey(tmp)){
        mp.get(tmp).add(allowed.get(i).charAt(allowed.get(i).length()-1));
      }else{
        mp.put(tmp, new ArrayList<>());
        mp.get(tmp).add(allowed.get(i).charAt(allowed.get(i).length()-1));
      }
    }
//    for(String s : mp.keySet()){
//      System.out.print(s + " " + mp.get(s) + "
");
//    }
    return helper(bottom, "", 0);
  }
  public boolean helper(String bottom, String up, int index){
    if(bottom.length() == 1){
      return true;
    }else {
      String tmp = bottom.substring(index, index+2);
      if(!mp.containsKey(tmp)) return false;
      List<Character> clist = mp.get(tmp);
      if(index+2 == bottom.length()){
        for(char x : clist){
          String finalup = up + String.valueOf(x);
          if(helper(finalup, "", 0)) return true;
        }
      }else {
        for(char x : clist){
          if(helper(bottom, up+String.valueOf(x), index+1)) return true;
        }
      }
    }
    return false;
  }
}