public class Solution {
/**
 * @param matrix a matrix of m x n elements
 * @return an integer list
 */
public List<Integer> spiralOrder(int[][] matrix) {
    // Write your code here
    List<Integer> result = new ArrayList<>();
    if (matrix == null) return result;
    int row = matrix.length;
    if (row == 0) return result;
    int column = matrix[0].length;
    tranverseEdge(result, 0, 0, row, column, matrix);
    return result;
}

private void tranverseEdge(List<Integer> result, int startRow, int startColumn, int rowLength, int columnLength, int[][] matrix) {
     if (columnLength == 0 || rowLength == 0) return; //这里是 || 不能是 && 比如 2 ＊ 4的
     if (rowLength == 1) { //当省一行或者一列要停，否则会重复遍历这一行或者一列。
        for (int i = startColumn; i < startColumn + columnLength; i++) {
            result.add(matrix[startRow][i]);
        }
        return;
    }
    if (columnLength == 1) {
        for (int i = startRow; i < startRow + rowLength; i++) {
            result.add(matrix[i][startColumn]);
        }
        return;
    }
    for (int i = startColumn; i < startColumn + columnLength - 1; i++) {
        result.add(matrix[startRow][i]);
    }
    for (int i = startRow; i < startRow + rowLength - 1; i++) {
        result.add(matrix[i][startColumn + columnLength - 1]);
    }
    for (int i = startColumn + columnLength - 1; i> startColumn;i--) {
        result.add(matrix[startRow + rowLength - 1][i]);
    }
    for (int i = startRow + rowLength - 1; i > startRow; i--) {
        result.add(matrix[i][startColumn]);
    }
    tranverseEdge(result, startRow + 1, startColumn + 1, rowLength - 2, columnLength - 2,matrix);
}
}