Question

Formatted question description: https://leetcode.ca/all/723.html

 723. Candy Crush

 This question is about implementing a basic elimination algorithm for Candy Crush.

 Given a 2D integer array board representing the grid of candy, different positive integers board[i][j] represent different types of candies.
 A value of board[i][j] = 0 represents that the cell at position (i, j) is empty.
 The given board represents the state of the game following the player's move.

 Now, you need to restore the board to a stable state by crushing candies according to the following rules:

     If three or more candies of the same type are adjacent vertically or horizontally, "crush" them all at the same time - these positions become empty.
     After crushing all candies simultaneously, if an empty space on the board has candies on top of itself, then these candies will drop until they hit a candy or bottom at the same time. (No new candies will drop outside the top boundary.)
     After the above steps, there may exist more candies that can be crushed. If so, you need to repeat the above steps.
     If there does not exist more candies that can be crushed (ie. the board is stable), then return the current board.
     You need to perform the above rules until the board becomes stable, then return the current board.


 Example:

 Input:
 board =
 [[110,5,112,113,114],[210,211,5,213,214],[310,311,3,313,314],[410,411,412,5,414],[5,1,512,3,3],[610,4,1,613,614],[710,1,2,713,714],[810,1,2,1,1],[1,1,2,2,2],[4,1,4,4,1014]]

 Output:
 [[0,0,0,0,0],[0,0,0,0,0],[0,0,0,0,0],[110,0,0,0,114],[210,0,0,0,214],[310,0,0,113,314],[410,0,0,213,414],[610,211,112,313,614],[710,311,412,613,714],[810,411,512,713,1014]]

 Explanation:


 Note:
     The length of board will be in the range [3, 50].
     The length of board[i] will be in the range [3, 50].
     Each board[i][j] will initially start as an integer in the range [1, 2000].

Algorithm

Don’t think too complicated (again!)

Look at only 3 digits horizontally and vertically, then mark them

O(m * n * (m + n)), each scan is m * n, and each direction can only collapse m times and n times at most

Code

Java

public class Candy_Crush {

    class Solution {
        public int[][] candyCrush(int[][] board) {

            boolean isCrushable = true;
            int m = board.length;
            int n = board[0].length;

            while (isCrushable) {
                isCrushable = false;

                // check every row
                for (int i = 0; i < m; i++) {
                    for (int j = 0; j < n - 2; j++) {
                        int val = Math.abs(board[i][j]);
                        if (val != 0 && Math.abs(board[i][j + 1]) == val
                            && Math.abs(board[i][j + 2]) == val) {

                            board[i][j] = board[i][j + 1] = board[i][j + 2] = -val; // mark as negative
                            isCrushable = true;
                        }
                    }
                }

                // check every column
                for (int i = 0; i < n; i++) {
                    for (int j = 0; j < m - 2; j++) {
                        int val = Math.abs(board[j][i]);
                        if (val != 0 && Math.abs(board[j + 1][i]) == val
                            && Math.abs(board[j + 2][i]) == val) {

                            board[j][i] = board[j + 1][i] = board[j + 2][i] = -val;
                            isCrushable = true;
                        }
                    }
                }

                // crushing, start from left-bottom
                for (int col = 0; col < n; col++) { // start from each col, because after crash it will fall down
                    int wr = m - 1;
                    for (int row = m - 1; row >= 0; row--) {
                        if (board[row][col] > 0)
                            board[wr--][col] = board[row][col];
                    }
                    while (wr >= 0) { // wipe the rest at higher band of each col
                        board[wr--][col] = 0;
                    }
                }
            }

            return board;
        }
    }

}

Java

class Solution {
    public int[][] candyCrush(int[][] board) {
        int rows = board.length, columns = board[0].length;
        boolean stable = false;
        while (!stable) {
            stable = true;
            List<int[]> removeList = new ArrayList<int[]>();
            for (int i = 0; i < rows; i++) {
                for (int j = 0; j < columns; j++) {
                    if (board[i][j] != 0) {
                        List<int[]> curRemoveList = search(board, i, j);
                        if (curRemoveList.size() > 0) {
                            removeList.addAll(curRemoveList);
                            stable = false;
                        }
                    }
                }
            }
            for (int[] cell : removeList)
                board[cell[0]][cell[1]] = 0;
            if (!stable)
                updateBoard(board);
        }
        return board;
    }

    public List<int[]> search(int[][] board, int row, int column) {
        List<int[]> curRemoveList = new ArrayList<int[]>();
        int value = board[row][column];
        if (value == 0)
            return curRemoveList;
        int rows = board.length, columns = board[0].length;
        int[][] directions = { {1, 0}, {0, 1} };
        for (int[] direction : directions) {
            List<int[]> directionRemoveList = new ArrayList<int[]>();
            int newRow = row + direction[0], newColumn = column + direction[1];
            while (newRow >= 0 && newRow < rows && newColumn >= 0 && newColumn < columns && board[newRow][newColumn] == value) {
                directionRemoveList.add(new int[]{newRow, newColumn});
                newRow += direction[0];
                newColumn += direction[1];
            }
            if (directionRemoveList.size() >= 2)
                curRemoveList.addAll(directionRemoveList);
        }
        if (curRemoveList.size() > 0)
            curRemoveList.add(new int[]{row, column});
        return curRemoveList;
    }

    public void updateBoard(int[][] board) {
        int rows = board.length, columns = board[0].length;
        for (int i = 0; i < columns; i++) {
            int index1 = rows - 1, index2 = rows - 1;
            while (index1 >= 0) {
                if (board[index1][i] == 0)
                    index1--;
                else
                    board[index2--][i] = board[index1--][i];
            }
            while (index2 >= 0)
                board[index2--][i] = 0;
        }
    }
}

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