Number Of Corner Rectangles

题目
Given a grid where each entry is only 0 or 1, find the number of corner rectangles.

A corner rectangle is 4 distinct 1s on the grid that form an axis-aligned rectangle. Note that only the corners need to have the value 1. Also, all four 1s used must be distinct.

Example 1:

Input: grid = 
[[1, 0, 0, 1, 0],
 [0, 0, 1, 0, 1],
 [0, 0, 0, 1, 0],
 [1, 0, 1, 0, 1]]
Output: 1
Explanation: There is only one corner rectangle, with corners grid[1][2], grid[1][4], grid[3][2], grid[3][4].

Note:
The number of rows and columns of grid will each be in the range [1, 200].
Each grid[i][j] will be either 0 or 1.
The number of 1s in the grid will be at most 6000.

答案

class Solution {
    public int countCornerRectangles(int[][] grid) {
        
        // Grid[i][j] is current point, grid[k][p] is diagonal point
        /*
            (i,j)             (i,p)
        
            (k,j)             (k,p)
        */
        int count = 0;
        for(int i = 0; i < grid.length; i++) {
            for(int j = 0; j < grid[0].length; j++) {
                for(int k = i + 1; k < grid.length && grid[i][j] == 1; k++) {
                    for(int p = j + 1; p < grid[0].length && grid[k][j] == 1; p++) {
                        count += grid[k][p] * grid[i][p];  
                    }
                }
            }
        }
        return count;
    }
}