741. Cherry Pickup

Posted 2021-02-16 ylzylz

741. 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, 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.
•  

解法：看起来和2d的Max path sum很像，所以想到也是DP的解法。但是不能用(0, 0) -> (n-1, n-1) (n-1, n-1) -> (0, 0) 两次DP：像，所以想

因为这个例子，最优解可以是从0，0走黄色到n,n，回头走粉色，可以得到所有1。

11100
00101
10100
00100
00111

如果用两次DP的方法的 从0，0走黄色到n,n，回头走最后一列粉色，无论如何会漏掉一个1，所以不是最优。

11100
00101
10100
00100
00111

所以我们需要同时有两个点的横纵坐标放入dp function。

题意说的是头到尾 moving right or down，从尾到头 moving left or up，但是算的时候可以用两个点同时从同个位置出发，便于计算。比如说 两个点 x, y 横纵坐标为(x1,x2) (y1,y2) ，同时从(n-1,n-1)出发，move moving left or up. 如果grid 长度为4：

(x1,x2) = {3,3}  (y1, y2) = {3, 3} 

next positions: 

{3,2} {3,2}  (x left, y left)
{2,3} {2,3}  (x up, y up)
{3,2} {2,3}  (x left, y up)
{2,3} {3,2}  (x up, y left)

注意到

x1 + x2 = y1 + y2 =>  y2 = x1 + x2 - y2, 所以 dp中可以用三个参数就好，减少计算量。

注意的点：

题意说如果没有路线要返回-1，所以如果越界或grid值为-1， return -1。但是注意要在(0,0)(0,0) init，如果没有init，(0,0)(0,0)的邻居都越界返回-1，所有值就都会变成-1。

 

 1 public int cherryPickup(int[][] grid) {
 2         HashMap<String, Integer> hm = new HashMap<String, Integer>();
 3         hm.put("0:0:0", grid[0][0]); // init
 4         return Math.max(0, helper(hm, grid.length - 1, grid[0].length - 1, grid.length - 1, grid));
 5 
 6     }
 7 
 8     int[][] delta = new int[][]{{0, -1, 0}, {-1, 0, -1}, {-1, 0, 0}, {0, -1, -1}};
 9     private int helper(HashMap<String, Integer> hm, int x1, int x2, int y1, int[][] grid) {
10         if (hm.containsKey(x1 + ":" + x2 + ":" + y1)) {
11             return hm.get(x1 + ":" + x2 + ":" + y1);
12         }
13         int y2 = x1 + x2 - y1;
14 
15         if (x1 < 0 || x1 >= grid.length || x2 < 0 || x2 >= grid[0].length || y1 < 0 || y1 >= grid.length
16             || y2 < 0 || y2 >= grid[0].length || grid[x1][x2] == -1 || grid[y1][y2] == -1) {
17             return -1;
18         }
19         int cur = 0;
20         if (x1 == y1 && x2 == y2) {
21             if (grid[x1][x2] == 1) {
22                 cur += 1;
23             }
24         } else { 
25             if (grid[x1][x2] == 1) {
26                 cur += 1;
27             }
28             if (grid[y1][y2] == 1) {
29                 cur += 1;
30             }
31         }
32         int neighbor = Integer.MIN_VALUE;
33         for (int k = 0; k < delta.length; k++) {
34             neighbor = Math.max(neighbor, helper(hm, x1 + delta[k][0], x2 + delta[k][1], y1 + delta[k][2], grid));
35         }
36         if (neighbor < 0) {
37             hm.put(x1 + ":" + x2 + ":" + y1, -1);
38             return -1;
39         }
40         hm.put(x1 + ":" + x2 + ":" + y1, neighbor + cur);
41         return neighbor + cur;
42     }