How do you traverse a matrix?

Most matrix problems are either index arithmetic or graph traversal in disguise. Clarify whether rows and columns can have different lengths, whether movement is four-directional or eight-directional, and whether the matrix can be modified in place.

function spiralOrder(matrix) {
  if (matrix.length === 0) return [];
  const result = [];
  let top = 0;
  let bottom = matrix.length - 1;
  let left = 0;
  let right = matrix[0].length - 1;

  while (top <= bottom && left <= right) {
    for (let col = left; col <= right; col++) result.push(matrix[top][col]);
    top += 1;
    for (let row = top; row <= bottom; row++) result.push(matrix[row][right]);
    right -= 1;
    if (top <= bottom) {
      for (let col = right; col >= left; col--) result.push(matrix[bottom][col]);
      bottom -= 1;
    }
    if (left <= right) {
      for (let row = bottom; row >= top; row--) result.push(matrix[row][left]);
      left += 1;
    }
  }

  return result;
}

Patterns to recognise

• Row-major scan: outer loop over rows, inner loop over columns
• Spiral: maintain top, bottom, left and right boundaries
• Diagonal: row and column indices move together or in opposite directions
• Grid BFS or DFS: generate neighbours from four directions
• In-place rotation: transpose, then reverse rows or columns

A common bug is assuming a square matrix. Always use matrix.length for rows and matrix[0].length for columns, and handle an empty matrix before reading the first row.