## README ¶

### 741. Cherry Pickup

#### 题目

In a N x N grid representing a field of cherries, each cell is one of three possible integers.

- 0 means the cell is empty, so you can pass through;
- 1 means the cell contains a cherry, that you can pick up and pass through;
- -1 means the cell contains a thorn that blocks your way.

Your task is to collect maximum number of cherries possible by following the rules below:

- Starting at the position (0, 0) and reaching (N-1, N-1) by moving right or down through valid path cells (cells with value 0 or 1);
- After reaching (N-1, N-1), returning to (0, 0) by moving left or up through valid path cells;
- When passing through a path cell containing a cherry, you pick it up and the cell becomes an empty cell (0);
- If there is no valid path between (0, 0) and (N-1, N-1), then no cherries can be collected.

Example 1:

```
Input: grid =
[[0, 1, -1],
[1, 0, -1],
[1, 1, 1]]
Output: 5
Explanation:
The player started at (0, 0) and went down, down, right right to reach (2, 2).
4 cherries were picked up during this single trip, and the matrix becomes [[0,1,-1],[0,0,-1],[0,0,0]].
Then, the player went left, up, up, left to return home, picking up one more cherry.
The total number of cherries picked up is 5, and this is the maximum possible.
```

Note:

- grid is an N by N 2D array, with 1 <= N <= 50.
- Each grid[i][j] is an integer in the set {-1, 0, 1}.
- It is guaranteed that grid[0][0] and grid[N-1][N-1] are not -1.

#### 解题思路

见程序注释

## Documentation ¶

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