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Problem solution · Java

Shortest Path in a Hidden Grid

Shortest Path in a Hidden Grid: a Java solution using breadth-first search. Learn the idea, check the complexity, and read the full code, with credit to walkccc LeetCode Solutions.

Technique
Breadth-first search
Source
walkccc LeetCode Solutions
Length
73 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

Breadth-first search

For Shortest Path in a Hidden Grid, the implementation explores reachable states in layers, which is the standard shape for unweighted shortest paths and minimum-step transitions.

  1. Model each valid configuration as a state and each legal move as an edge.
  2. Seed the queue with the starting state and mark it immediately.
  3. Expand each state once, recording distance or reachability for unseen neighbours.

Code notes

  • 73 lines of Java from the credited upstream file 1778.java.
  • The implementation visibly relies on sequence storage, work queue.
  • 4 loop blocks detected, together with recursive traversal.

Complexity

Verify that each state and transition is processed only a bounded number of times; that determines the traversal cost.

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 codeShortest Path in a Hidden Grid · JavaJava
Use this to learn the idea, then write your own version.
/** * // This is the GridMaster's API interface. * // You should not implement it, or speculate about its implementation * class GridMaster { *   boolean canMove(char direction); *   void move(char direction); *   boolean isTarget(); * } */ enum Grid { UNVISITED, START, TARGET, BLOCKED, EMPTY } class Solution {  public int findShortestPath(GridMaster master) {    final int m = 501;    final int startX = m;    final int startY = m;    Grid[][] grid = new Grid[m * 2][m * 2];    Arrays.stream(grid).forEach(A -> Arrays.fill(A, Grid.UNVISITED));     // Build the grid information by DFS.    dfs(master, grid, startX, startY);     Queue<Pair<Integer, Integer>> q = new ArrayDeque<>(List.of(new Pair<>(startX, startY)));    grid[startX][startY] = Grid.BLOCKED;     // Find the steps by BFS.    for (int step = 1; !q.isEmpty(); ++step)      for (int sz = q.size(); sz > 0; --sz) {        final int i = q.peek().getKey();        final int j = q.poll().getValue();        for (int[] dir : DIRS) {          final int x = i + dir[0];          final int y = j + dir[1];          if (grid[x][y] == Grid.TARGET)            return step;          if (grid[x][y] == Grid.BLOCKED)            continue;          grid[x][y] = Grid.BLOCKED;          q.offer(new Pair<>(x, y));        }      }     return -1;  }   private static final int[][] DIRS = {{0, 1}, {1, 0}, {0, -1}, {-1, 0}};  private static final char[] charTable = {'R', 'D', 'L', 'U'};   private void dfs(GridMaster master, Grid[][] grid, int i, int j) {    if (grid[i][j] != Grid.UNVISITED)      return;    if (master.isTarget())      grid[i][j] = Grid.TARGET;    else      grid[i][j] = Grid.EMPTY;     for (int k = 0; k < 4; ++k) {      final int x = i + DIRS[k][0];      final int y = j + DIRS[k][1];      final char d = charTable[k];      final char undoD = charTable[(k + 2) % 4];      if (master.canMove(d)) {        master.move(d);        dfs(master, grid, x, y);        master.move(undoD);      } else {        grid[x][y] = Grid.BLOCKED;      }    }  }}

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