There is a interview question on bfe.dev about decode-message. The link is below

https://bigfrontend.dev/problem/decode-message

Your are given a 2-D array of characters. There is a hidden message in it.

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I B C A L K A
D R F C A E A
G H O E L A D

col and row are marked with number which start from 0. The number of col is 0 ~ 6 ,and the number of row is 0~2
The way to collect the message is as follows

  1. start at top left
  2. move diagonally down right
  3. when cannot move any more, try to switch to diagonally up right
  4. when cannot move any more, try switch to diagonally down right, repeat 3
  5. stop when cannot neither move down right or up right. the character on the path is the message
    for the input above, IROCLED should be returned
    notes:
    if no characters could be collected, return empty string

now we have a matrix of 3 rows , 7 columns.
we’ll start at coordinates [0,0] . and next we’ll move to [1,1] ,the rest coordinates will be [2,2]
[3,1] [4,0] [5,1] [6,2]

so in the function decode,it recieves a 2D array

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/**
 * @param {string[][]} message
 * @return {string}
 */
function decode(message) {
  // your code here
}
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message=[
[I, B, C, A, L, K, A],
[D, R, F, C, A, E, A],
[G, H, O, E, L,A, D]
]

The value of variable rows will be the lenght of messsage ,that’s message.length ,and it is 3 .
The value of varibable columns will be message[0].length ,and it’s 7 .
if the rows or columns is 0 , the function should return.
and we set the default value of variable result as ‘’ ;
we set row as 0 and col as 0;
we use a while loop to get every col in columns,codes are blow

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/**
 * @param {string[][]} message
 * @return {string}
 */
function decode(message) {
  if(message.length === 0)return '';
  if(message[0].length===0) return '';
  let rows = message.length;
  let cols = message[0].length;

  let result = '';
  let row = 0;
  let col = 0;
}

we use a while loop to get every col in columns
so in the loop col will alway plus 1 ,row will plus the direction which name is directionY ,when row is bigger than 2 ,that is row> rows-1 ,directionY should be -1 ,row have to minus 2

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if(row>rows-1){
  directonY = -1;
  row -=2
}

if row is small than 0 ,we in the top of matrix , In this case directionY should be 1 ,row have to add 2

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if(row<0){
  directonY = 1;
  row +=2
}

row also have a limitation ,as we doing row-=2 ,and row +=2; we can’t let row out of the matrix .so row must bigger than -1 ,and < rows.
the function is blow ,and it passed the test.

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/**
 * @param {string[][]} message
 * @return {string}
 */
function decode(message) {
  if(message.length === 0)return '';
  if(message[0].length===0) return '';
  let rows = message.length;
  let cols = message[0].length;

  let result = '';
  let row = 0;
  let col = 0;
  let directionY = 1;
  /**
  when col = 3 ,row =  3  
  rows =3  , 3 > 3-1
  */
  while(col < cols && row >-1 && row<rows){
    result += message[row][col]
    col+=1
    row +=directionY
  
  if(row>rows-1){
    directionY = -1;
    row -= 2
  }else if(row<0){
     directionY = 1;
    row += 2
  }
  }
  return result

  
}