LeetCode 54. Spiral Matrix

54. Spiral Matrix

Difficulty:: Medium

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]

Solution

Language: Java

class Solution {
    public List<Integer> spiralOrder(int[][] matrix) {
        List<Integer> result = new ArrayList<>();
        if (matrix == null || matrix.length == 0) {
            return result;
        }
        int m = matrix.length;
        int n = matrix[0].length;
        int left = 0, up = 0, right = n - 1, down = m - 1;
        while (left <= right && up <= down) {
            for (int i = left; i <= right; i++) {
                result.add(matrix[up][i]);
            }
            up++;
            for (int i = up; i <= down; i++) {
                result.add(matrix[i][right]);
            }
            right--;
            if (up <= down && left <= right) {
                for (int i = right; i >= left; i--) {
                    result.add(matrix[down][i]);
                }
                down--;
                
                for (int i = down; i >= up; i--) {
                    result.add(matrix[i][left]);
                }
                left++;
            }
        }
        return result;
    }
}