题解 | #螺旋矩阵#

import java.util.ArrayList;
public class Solution {
    public ArrayList<Integer> spiralOrder(int[][] matrix) {
        if (matrix == null) return null;
        ArrayList<Integer> res = new ArrayList<>();
        if (matrix.length == 0) return res;

        int top = 0;
        int bottom = matrix.length - 1;
        int left = 0;
        int right = matrix[0].length - 1;
        while (top <= bottom && left <= right) {
            // left top -> right top
            for (int i = left; i <= right; i++) {
                res.add(matrix[top][i]);
            }
            top++;

            // right top -> right bottom
            for (int i = top; i <= bottom; i++) {
                res.add(matrix[i][right]);
            }
            right--;

            // 奇数行、偶数列的时候有问题
            if (top > bottom || left > right) break;

            // right bottom -> left bottom
            for (int i = right; i >= left; i--) {
                res.add(matrix[bottom][i]);
            }
            bottom--;

            // left bottom -> left top
            for (int i = bottom; i >= top; i--) {
                res.add(matrix[i][left]);
            }
            left++;
        }

        return res;
    }
}

解题思想：确定四个边界值，主键缩小边界值，注意特殊情况的奇数行偶数列需要特殊处理。