Coding Challenge Practice - Question 83

The task is to implement a decode function to reveal the hidden message in a given 2-D array of characters.

The boilerplate code

function decode(message) {
  // your code here
}

If there's no hidden message, return an empty string

 if (!message || message.length === 0 || message[0].length === 0) {
    return '';
  }

Define the grid's dimensions

const rows = message.length;
const cols = message[0].length;

Define variables to track the position and direction while moving through the grid

let row = 0;
let col = 0;
let goingDown = true;
let result = '';

When goingDown is true, it means the direction of movement is downwards to the right. If it is false, the direction of movement is upwards to the left. While moving across the grid, each character in a visited cell is collected and appended to the result.

while (col < cols && row >= 0 && row < rows) {
    result += message[row][col];
}

Move down one row, and right one column as long as possible

if (row + 1 < rows && col + 1 < cols) {
  row++;
  col++;
}

When the bottom or the right edge is reached, move up one row and right one column

 else {
        goingDown = false;
        if (col + 1 < cols) {
          col++;
          row--;
        } else {
          break;
        }
      }
    }

When the top or right edge is reached, move down one row and right one column

else {
      if (row - 1 >= 0 && col + 1 < cols) {
        row--;
        col++;
      } else {
        goingDown = true;
        if (col + 1 < cols) {
          col++;
          row++;
        } else {
          break;
        }
      }
    }

Stop when moving to the right is no longer possible

return result

The final code

function decode(message) {
  // your code here
   if (!message || message.length === 0 || message[0].length === 0) {
    return '';
  }

  const rows = message.length;
  const cols = message[0].length;

  let row = 0;
  let col = 0;
  let goingDown = true;
  let result = '';

  while(col < cols && row >= 0 && row < rows) {
    result += message[row][col];

    if(goingDown) {
      if(row + 1 < rows && col + 1 < cols) {
        row++;
        col++;
      } else {
        goingDown = false;
        if(col + 1 < cols) {
          col++;
          row--;
        } else {
          break;
        }
      }
    } else {
      if(row - 1 >= 0 && col + 1 < cols) {
        row--;
        col++;
      } else {
        goingDown = true;
        if(col + 1 < cols) {
          col++;
          row++;
        } else {
          break;
        }
      }
    }
  }
  return result;
}

That's all folks!