Spiral Matrix

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

For example,
Given the following matrix:

 

public class Solution {
    public List<Integer> spiralOrder(int[][] matrix) {
    	List<Integer> res = new ArrayList<>();
    	if (matrix == null || matrix.length == 0) {
    		return res;
    	}
    	int tR = 0;
    	int tC = 0;
    	int dR = matrix.length-1;
    	int dC = matrix[0].length-1;
    	while (tR <= dR && tC <= dC) {
    		printEdge(matrix, tR++, tC++, dR--, dC--, res);
    	}
    	return res;
    }

	private void printEdge(int[][] matrix, int tR, int tC, int dR, int dC, List<Integer> res) {
		// TODO Auto-generated method stub
		if (tR == dR) {
			for (int i = tC; i <= dC; i++) {
				res.add(matrix[tR][i]);
			}
		} else if (tC == dC) {
			for (int i = tR; i <= dR; i++) {
				res.add(matrix[i][tC]);
			}
		} else {
			int curC = tC;
			int curR = tR;
			while (curC != dC) {
				res.add(matrix[tR][curC]);
				curC++;
			}
			while (curR != dR) {
				res.add(matrix[curR][dC]);
				curR++;
			}
			while (curC != tC) {
				res.add(matrix[dR][curC]);
				curC--;
			}
			while (curR != tR) {
				res.add(matrix[curR][tC]);
				curR--;
			}
		}
	}
}