54. Spiral Matrix

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Problem

Given a matrix of m x n elements (m rows, n columns), return all elements of the matrix in spiral order.

Example 1:

Input:
[
 [ 1, 2, 3 ],
 [ 4, 5, 6 ],
 [ 7, 8, 9 ]
]
Output: [1,2,3,6,9,8,7,4,5]

Example 2:

Input:
[
  [1, 2, 3, 4],
  [5, 6, 7, 8],
  [9,10,11,12]
]
Output: [1,2,3,4,8,12,11,10,9,5,6,7]

Code

class Solution {
    public List<Integer> spiralOrder(int[][] matrix) {
        List<Integer> res = new ArrayList<Integer>();

        if (matrix.length == 0) {
            return res;
        }

        int rowBegin = 0;
        int rowEnd = matrix.length-1;
        int colBegin = 0;
        int colEnd = matrix[0].length - 1;

        while (rowBegin <= rowEnd && colBegin <= colEnd) {
            // Traverse Right
            for (int j = colBegin; j <= colEnd; j++) {
                res.add(matrix[rowBegin][j]);
            }
            rowBegin++;

            // Traverse Down
            for (int j = rowBegin; j <= rowEnd; j++) {
                res.add(matrix[j][colEnd]);
            }
            colEnd--;

            if (rowBegin <= rowEnd) {
                // Traverse Left
                for (int j = colEnd; j >= colBegin; j--) {
                    res.add(matrix[rowEnd][j]);
                }
            }
            rowEnd--;

            if (colBegin <= colEnd) {
                // Traverse Up
                for (int j = rowEnd; j >= rowBegin; j--) {
                    res.add(matrix[j][colBegin]);
                }
            }
            colBegin++;
        }

        return res;
    }
}