1.7 Matrix Traversal

Demo: Spiral Matrix — LC 54 (Medium)

Statement (Demo): Given an m × n integer matrix, return all its elements in spiral order (right → down → left → up). Example: [[1,2,3],[4,5,6],[7,8,9]] → [1,2,3,6,9,8,7,4,5] | ⚡ m,n ≤ 10

Approach: Keep four boundaries (top/bottom/left/right). Walk one layer, then pull the boundaries in. Check the boundaries before the bottom and left passes, or a single inner row/column gets counted twice.

function spiralOrder(matrix) {
  const res = [];
  let top = 0,
    bottom = matrix.length - 1;
  let left = 0,
    right = matrix[0].length - 1;
  while (top <= bottom && left <= right) {
    for (let j = left; j <= right; j++) res.push(matrix[top][j]);
    top++;
    for (let i = top; i <= bottom; i++) res.push(matrix[i][right]);
    right--;
    if (top <= bottom) {
      for (let j = right; j >= left; j--) res.push(matrix[bottom][j]);
      bottom--;
    }
    if (left <= right) {
      for (let i = bottom; i >= top; i--) res.push(matrix[i][left]);
      left++;
    }
  }
  return res;
}

Complexity: Time O(m·n), Space O(1) (excluding output)