LeetCode/矩阵/1. 螺旋矩阵.md

# 螺旋矩阵

## 题目

给你一个 m 行 n 列的矩阵 matrix ，请按照 顺时针螺旋顺序 ，返回矩阵中的所有元素。

 

示例 1：


输入：matrix = [[1,2,3],[4,5,6],[7,8,9]]
输出：[1,2,3,6,9,8,7,4,5]
示例 2：


输入：matrix = [[1,2,3,4],[5,6,7,8],[9,10,11,12]]
输出：[1,2,3,4,8,12,11,10,9,5,6,7]
 

提示：

m == matrix.length
n == matrix[i].length
1 <= m, n <= 10
-100 <= matrix[i][j] <= 100

## 思路

- 模拟
  - 方向
    - →↓←↑
  - 边界
    - `up`
    - `down`
    - `left`
    - `right`

## 代码

```java
class Solution {
    public List<Integer> spiralOrder(int[][] matrix) {
        // Init: up, down, left, right
        int up = 0;
        int down = matrix.length - 1;
        int left = 0;
        int right = matrix[0].length - 1;
        // Init: ans
        List<Integer> ans = new ArrayList<>();
        int tar = matrix.length * matrix[0].length;
        // Traverse
        while (left <= right && up <= down) {
            for (int i = left; i <= right; i++) {
                ans.add(matrix[up][i]);
            }
            up ++;

            for (int i = up; i <= down; i++) {
                ans.add(matrix[i][right]);
            }
            right --;
            
            if (up <= down) {
                for (int i = right; i >= left; i--) {
                    ans.add(matrix[down][i]);
                }
            }
            down --;
            
            if (left <= right) {
                for (int i = down; i >= up; i--) {
                    ans.add(matrix[i][left]);
                }
            }
            left ++;

        }

        return ans;
    }
}
```