public String toString$()$

Returns a String representing the characters stored in grid in a format that is

easy to read, that exactly matches the format given below.

o $($new LetterCrush$(4,3,$"AACADBBDCD"$)).$toString$()$ must return

Note the row number at the end of the $2^{nd}$ to $4^{th}$ lines, and the column numbers in

the last row. The column indices will always be one$-$digit values $($grids with widths

greater than nine will not be tested$).$

public boolean isStable$()$

Returns false if there is any position in grid where a non$-$EMPTY character is

above an EMPTY character $($a grid entry with a smaller row than a grid entry with

a larger row in the same column $-$ thus, for the example given above in the

description of the toString$()$ method, isStable$()$ would return false$).$

Otherwise, returns true.public Line longestLine$()$

O Returns null if there does not exist in grid a line of at least $3$ adjacent matching letters.

Otherwise, returns a uniquely defined longest line in grid that contains adjacent

matching letters. To make this method unambiguous, preferences must be

specified to select among equally long lines of adjacent matching letters. Use

these preferences for this method:

A horizontal line has preference over a vertical one of the same length.

If two horizontal lines have the longest length, then we give preference to

the line appearing in the lowest row $($largest index$)$; if both lines are in the

same row, we prefer the line that appears to the left.

If two vertical lines have the longest length, we give preference to the line

that appears in the leftmost column; if both lines appear in the same

column, we prefer the line the appears at the bottom.

For example, if there are three lines with the maximum length and none of them

are horizontal, and among these lines two of them are in the same column but to

the left of the third line, then the line in the left column that appears in the lowest

row will be the one that is returned.

See the pseudocode below for details.public void cascade$()$

O This method iteratively removes the longest line of adjacent matching letters, of

length at least $3$ and repeatedly invokes the applyGravity method to shift letters

from grid to the empty spaces until no more shifts are possible $($you can check

this by invoking method isStable$).$ The process is repeated until the longest line

of adjacent matching letters has a length less than three.