LeetCode 764. Largest Plus Sign

原题链接在这里：https://leetcode.com/problems/largest-plus-sign/

题目：

In a 2D grid from (0, 0) to (N-1, N-1), every cell contains a 1, except those cells in the given list mines which are 0. What is the largest axis-aligned plus sign of 1s contained in the grid? Return the order of the plus sign. If there is none, return 0.

An "axis-aligned plus sign of 1s of order k" has some center grid[x][y] = 1 along with 4 arms of length k-1 going up, down, left, and right, and made of 1s. This is demonstrated in the diagrams below. Note that there could be 0s or 1s beyond the arms of the plus sign, only the relevant area of the plus sign is checked for 1s.

Examples of Axis-Aligned Plus Signs of Order k:

Order 1:
000
010
000

Order 2:
00000
00100
01110
00100
00000

Order 3:
0000000
0001000
0001000
0111110
0001000
0001000
0000000

Example 1:

Input: N = 5, mines = [[4, 2]]
Output: 2
Explanation:
11111
11111
11111
11111
11011
In the above grid, the largest plus sign can only be order 2.  One of them is marked in bold.

Example 2:

Input: N = 2, mines = []
Output: 1
Explanation:
There is no plus sign of order 2, but there is of order 1.

Example 3:

Input: N = 1, mines = [[0, 0]]
Output: 0
Explanation:
There is no plus sign, so return 0.

Note:

1. N will be an integer in the range [1, 500].
2. mines will have length at most 5000.
3. mines[i] will be length 2 and consist of integers in the range [0, N-1].
4. (Additionally, programs submitted in C, C++, or C# will be judged with a slightly smaller time limit.)

题解：

For each cell, plus sign is its min of left, right, up and down reach.

We could use brute force. It would take O(N) for each cell and totally O(N^3).

Otherwise, we could iterate each row left to right, calculate farest left reach for each cell. Unless cell is 0, keep accumlate length.

Iterate each row right to left, calculate farest right reach for each cell.

Same for columns to get farest up and down reach.

Get the minimum from farest left, right, up and down reach. Update final result.

Time Complexity: O(N^2).

Space: O(N^2).

AC Java:

class Solution {
    public int orderOfLargestPlusSign(int N, int[][] mines) {
        if(N <= 0){
            return 0;
        }
        
        int [][] matrix = new int[N][N];
        for(int i = 0; i<N; i++){
            Arrays.fill(matrix[i], N);
        }
        
        for(int [] mine : mines){
            matrix[mine[0]][mine[1]] = 0;
        }
        
        for(int i = 0; i<N; i++){
            for(int col = 0, l = 0; col<N; col++){
                l = matrix[i][col] == 0 ? 0 : l+1;
                matrix[i][col] = Math.min(matrix[i][col], l);
            }
            
            for(int col = N-1, r = 0; col>=0; col--){
                r = matrix[i][col] == 0 ? 0 : r+1;
                matrix[i][col] = Math.min(matrix[i][col], r);
            }
            
            for(int row = 0, u = 0; row<N; row++){
                u = matrix[row][i] == 0 ? 0 : u+1;
                matrix[row][i] = Math.min(matrix[row][i], u);
            }
            
            for(int row = N-1, d = 0; row>=0; row--){
                d = matrix[row][i] == 0 ? 0 : d+1;
                matrix[row][i] = Math.min(matrix[row][i], d);
            }
        }
        
        int res = 0;
        for(int i = 0; i<N; i++){
            for(int j = 0; j<N; j++){
                res = Math.max(res, matrix[i][j]);
            }
        }
        
        return res;
    }
}