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Problem solution · Java

Zuma Game

Zuma Game: a Java solution using depth-first search. Learn the idea, check the complexity, and read the full code, with credit to walkccc LeetCode Solutions.

Technique
Depth-first search
Source
walkccc LeetCode Solutions
Length
55 lines
Start with the idea.

Try the problem first. If you get stuck, read the approach below, then write your own solution. The full code is at the bottom.

Approach

Depth-first search

For Zuma Game, the implementation follows one branch at a time, making it suitable for components, trees, backtracking, or dependency exploration.

  1. Define the state carried into one recursive or stack frame.
  2. Mark or choose the current state before exploring children.
  3. Combine child results or undo the choice when the branch finishes.

Code notes

  • 55 lines of Java from the credited upstream file 488.java.
  • The implementation visibly relies on hash lookup, ordered lookup.
  • 5 loop blocks detected, together with recursive traversal.

Complexity

Count unique states for graph traversal; for backtracking, count the branching factor and maximum depth.

Check the problem constraints before deciding whether this complexity will pass.

Source

Code and credit

This code comes from walkccc LeetCode Solutions by P.-Y. Chen (walkccc) and is used under the MIT licence.

Open problem ↗Original source ↗
Full codeZuma Game · JavaJava
Use this to learn the idea, then write your own version.
class Solution {  public int findMinStep(String board, String hand) {    Map<String, Integer> mem = new HashMap<>();    final int ans = dfs(board + '#', hand, mem);    return ans == Integer.MAX_VALUE ? -1 : ans;  }   private int dfs(String board, final String hand, Map<String, Integer> mem) {    final String hashKey = board + '#' + hand;    if (mem.containsKey(hashKey))      return mem.get(hashKey);    board = deDup(board);    if (board.equals("#"))      return 0;     Set<Character> boardSet = new HashSet<>();    for (final char c : board.toCharArray())      boardSet.add(c);     StringBuilder sb = new StringBuilder();    for (final char h : hand.toCharArray())      if (boardSet.contains(h))        sb.append(h);    final String hs = sb.toString();    if (sb.length() == 0) // infeasible      return Integer.MAX_VALUE;     int ans = Integer.MAX_VALUE;     for (int i = 0; i < board.length(); ++i)      for (int j = 0; j < hs.length(); ++j) {        // Place hs[j] in board[i].        final String newHand = hs.substring(0, j) + hs.substring(j + 1);        String newBoard = board.substring(0, i) + hs.charAt(j) + board.substring(i);        final int res = dfs(newBoard, newHand, mem);        if (res < Integer.MAX_VALUE)          ans = Math.min(ans, 1 + res);      }     mem.put(hashKey, ans);    return ans;  }   private String deDup(String board) {    int start = 0; // the start index of a color sequenece    for (int i = 0; i < board.length(); ++i)      if (board.charAt(i) != board.charAt(start)) {        if (i - start >= 3)          return deDup(board.substring(0, start) + board.substring(i));        start = i; // Meet a new sequence.      }    return board;  }}

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