[Swift]LeetCode741. 摘樱桃 | Cherry Pickup

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,then no cherries can be collected.

Example 1:

Input: grid =
[[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,[0,0]].
Then,the player went left,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 {-1,1}.
It is guaranteed that grid[0][0] and grid[N-1][N-1] are not -1.

一个N x N的网格(grid) 代表了一块樱桃地，每个格子由以下三种数字的一种来表示：

0 表示这个格子是空的，所以你可以穿过它。
1 表示这个格子里装着一个樱桃，你可以摘到樱桃然后穿过它。
-1 表示这个格子里有荆棘，挡着你的路。

你的任务是在遵守下列规则的情况下，尽可能的摘到最多樱桃：

从位置 (0,0) 出发，最后到达 (N-1,N-1) ，只能向下或向右走，并且只能穿越有效的格子（即只可以穿过值为0或者1的格子）；
当到达 (N-1,N-1) 后，你要继续走，直到返回到 (0,0) ，只能向上或向左走，并且只能穿越有效的格子；
当你经过一个格子且这个格子包含一个樱桃时，你将摘到樱桃并且这个格子会变成空的（值变为0）；
如果在 (0,0) 和 (N-1,N-1) 之间不存在一条可经过的路径，则没有任何一个樱桃能被摘到。

示例 1:

输入: grid =
[[0,1]]
输出: 5
解释： 
玩家从（0,0）点出发，经过了向下走，向下走，向右走，向右走，到达了点(2,2)。
在这趟单程中，总共摘到了4颗樱桃，矩阵变成了[[0,0]]。
接着，这名玩家向左走，向上走，向上走，向左走，返回了起始点，又摘到了1颗樱桃。
在旅程中，总共摘到了5颗樱桃，这是可以摘到的最大值了。

说明:

grid 是一个 N * N 的二维数组，N的取值范围是1 <= N <= 50。
每一个 grid[i][j] 都是集合 {-1,1}其中的一个数。
可以保证起点 grid[0][0] 和终点 grid[N-1][N-1] 的值都不会是 -1。

384ms

 1 class Solution {
 2     func cherryPickup(_ grid: [[Int]]) -> Int {
 3         let n = grid.count
 4         var dp = Array(repeating: Array(repeating: Array(repeating: -1,count: n+1),count: n+1)
 5         
 6         dp[1][1][1] = grid[0][0]
 7         for x1 in 1...n {
 8             for y1 in 1...n {
 9                 for x2 in 1...n {
10                     let y2 = x1 + y1 - x2;
11                     if dp[x1][y1][x2] > 0 ||
12                         y2 < 1 || 
13                         y2 > n || 
14                         grid[x1 - 1][y1 - 1] == -1 || 
15                         grid[x2 - 1][y2 - 1] == -1 {
16                         continue
17                     }
18                     let cur = max(max(dp[x1 - 1][y1][x2],dp[x1 - 1][y1][x2 - 1]),19                                   max(dp[x1][y1 - 1][x2],dp[x1][y1 - 1][x2 - 1]))
20                     if cur < 0 {
21                         continue
22                     }
23                     dp[x1][y1][x2] = cur + grid[x1 - 1][y1 - 1]
24                     if x1 != x2 {
25                         dp[x1][y1][x2] += grid[x2 - 1][y2 - 1]
26                     }
27                 }
28             }
29         }
30         return dp[n][n][n] < 0 ? 0 : dp[n][n][n]
31     }
32 }

740ms

 1 class Solution {
 2     func cherryPickup(_ grid: [[Int]]) -> Int {
 3         let length = grid.count
 4         guard length != 0 && length == grid.first!.count && length >= 1 && length <= 50 && grid[0][0] != -1 && grid[length - 1][length - 1] != -1 else {
 5             return 0
 6         }
 7         var pickUpCount = Array(repeating: Array(repeating: -1,count: length),count: length)
 8         pickUpCount[0][0] = grid[0][0]
 9         if length > 1 {
10             for step in 1 ... (length - 1) * 2 {
11                 let xMax = min(length - 1,step),xMin = max(0,step - (length - 1))
12                 for x1 in stride(from: xMax,through: xMin,by: -1) {
13                     for x2 in stride(from: xMax,by: -1) {
14                         let y1 = step - x1,y2 = step - x2
15                         if grid[x1][y1] == -1 || grid[x2][y2] == -1 {
16                             pickUpCount[x1][x2] = -1
17                             continue
18                         }
19                         if y1 > 0 && x2 > 0 {
20                             pickUpCount[x1][x2] = max(pickUpCount[x1][x2],pickUpCount[x1][x2 - 1])
21                         }
22                         if x1 > 0 && y2 > 0 {
23                             pickUpCount[x1][x2] = max(pickUpCount[x1][x2],pickUpCount[x1 - 1][x2])
24                         }
25                         if x1 > 0 && x2 > 0 {
26                             pickUpCount[x1][x2] = max(pickUpCount[x1][x2],pickUpCount[x1 - 1][x2 - 1])
27                         }
28                         if pickUpCount[x1][x2] == -1 {
29                             continue
30                         }
31                         if x1 == x2 {
32                             pickUpCount[x1][x2] += grid[x1][y1]
33                         } else {
34                             pickUpCount[x1][x2] += grid[x1][y1] + grid[x2][y2]
35                         }
36                     }
37                 }
38             }
39         }
40         return max(pickUpCount[length - 1][length - 1],0)
41     }
42 }

Runtime: 876 ms

Memory Usage: 19 MB

 1 class Solution {
 2     func cherryPickup(_ grid: [[Int]]) -> Int {
 3         var n:Int = grid.count
 4         var mx:Int = 2 * n - 1
 5         var dp:[[Int]] = [[Int]](repeating:[Int](repeating:-1,count:n),count:n)
 6         dp[0][0] = grid[0][0]
 7         for k in 1..<mx
 8         {
 9             for i in stride(from:n - 1,through:0,by:-1)
10             {
11                 for p in stride(from:n - 1,by:-1)
12                 {
13                     var j:Int = k - i
14                     var q:Int = k - p
15                     if j < 0 || j >= n || q < 0 || q >= n || grid[i][j] < 0 || grid[p][q] < 0
16                     {
17                         dp[i][p] = -1
18                         continue
19                     }
20                     if i > 0 {dp[i][p] = max(dp[i][p],dp[i - 1][p])}
21                     if p > 0 {dp[i][p] = max(dp[i][p],dp[i][p - 1])}
22                     if i > 0 && p > 0 {dp[i][p] = max(dp[i][p],dp[i - 1][p - 1])}
23                     if dp[i][p] >= 0 {dp[i][p] += grid[i][j] + (i != p ? grid[p][q] : 0)}
24                 }
25             }
26         }
27         return max(dp[n - 1][n - 1],0)
28     }
29 }

[Swift]LeetCode741. 摘樱桃 | Cherry Pickup

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