public class Solution {

public List<Integer> spiralOrder(int[][] matrix) {

List<Integer> res = new ArrayList<>();

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

int rowBegin = 0, rowEnd = matrix.length - 1;

int colBegin = 0, 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;

}

}