Matrix Traversal and Manipulation

Written by Swapnil RajaPublished 28 Aug 2026

Grid problems are graph problems with implicit edges. Before any of that, though, you have to create the grid correctly - and the obvious way is wrong.

The bug to get out of the way first

Every row is the same array

javascript

// WRONG — fill() copies the reference, not the array.
const grid = new Array(3).fill(new Array(3).fill(0))
grid[0][0] = 1
console.log(grid)
// [[1,0,0], [1,0,0], [1,0,0]]  — all three rows changed

// RIGHT — a fresh array per row.
const grid2 = Array.from({ length: 3 }, () => new Array(3).fill(0))
grid2[0][0] = 1
console.log(grid2)
// [[1,0,0], [0,0,0], [0,0,0]]

PreviousBit Manipulation Basics NextRecognising the Pattern

Rotate 90 degrees in place

javascript

function rotate(matrix) {
  const n = matrix.length

  // Transpose: swap across the diagonal.
  for (let i = 0; i < n; i++) {
    for (let j = i + 1; j < n; j++) {
      ;[matrix[i][j], matrix[j][i]] = [matrix[j][i], matrix[i][j]]
    }
  }

  // Then reverse each row.
  for (const row of matrix) row.reverse()

  return matrix
}

console.log(rotate([[1,2,3], [4,5,6], [7,8,9]]))
// [[7,4,1], [8,5,2], [9,6,3]]

Spiral order

javascript

function spiralOrder(matrix) {
  if (matrix.length === 0) return []

  const out = []
  let top = 0
  let bottom = matrix.length - 1
  let left = 0
  let right = matrix[0].length - 1

  while (top <= bottom && left <= right) {
    for (let c = left; c <= right; c++) out.push(matrix[top][c])
    top++

    for (let r = top; r <= bottom; r++) out.push(matrix[r][right])
    right--

    // Guard: the row may already have been consumed above.
    if (top <= bottom) {
      for (let c = right; c >= left; c--) out.push(matrix[bottom][c])
      bottom--
    }

    if (left <= right) {
      for (let r = bottom; r >= top; r--) out.push(matrix[r][left])
      left++
    }
  }

  return out
}

console.log(spiralOrder([[1,2,3], [4,5,6], [7,8,9]]))
// [1,2,3,6,9,8,7,4,5]

Counting islands

javascript

function countIslands(grid) {
  if (grid.length === 0) return 0

  const rows = grid.length
  const cols = grid[0].length
  let count = 0

  function sink(r, c) {
    if (r < 0 || r >= rows || c < 0 || c >= cols) return
    if (grid[r][c] !== "1") return

    grid[r][c] = "0"   // mark visited by overwriting

    sink(r + 1, c)
    sink(r - 1, c)
    sink(r, c + 1)
    sink(r, c - 1)
  }

  for (let r = 0; r < rows; r++) {
    for (let c = 0; c < cols; c++) {
      if (grid[r][c] === "1") {
        count++
        sink(r, c)
      }
    }
  }

  return count
}

Matrix Traversal and Manipulation

The bug to get out of the way first

Transpose and rotate

Spiral traversal

Grids as graphs