Number Of Corner Rectangles: Accepted

Author: gouthampradhan

README.md

@@ -140,6 +140,7 @@ My accepted leetcode solutions to some of the common interview problems.
 - [Combination Sum IV](problems/src/dynamic_programming/CombinationSumIV.java) (Medium)
 - [Paint House II](problems/src/dynamic_programming/PaintHouseII.java) (Hard)
 - [Split Array Largest Sum](problems/src/dynamic_programming/SplitArrayLargestSum.java) (Hard)
+- [Number Of Corner Rectangles](problems/src/dynamic_programming/CornerRectangles.java) (Medium)
 
 #### [Greedy](problems/src/greedy)
 

problems/src/dynamic_programming/CornerRectangles.java

@@ -0,0 +1,71 @@
+package dynamic_programming;
+
+/**
+ * Created by gouthamvidyapradhan on 26/12/2017.
+ * 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].
+ Example 2:
+ Input: grid =
+ [[1, 1, 1],
+ [1, 1, 1],
+ [1, 1, 1]]
+ Output: 9
+ Explanation: There are four 2x2 rectangles, four 2x3 and 3x2 rectangles, and one 3x3 rectangle.
+ Example 3:
+ Input: grid =
+ [[1, 1, 1, 1]]
+ Output: 0
+ Explanation: Rectangles must have four distinct corners.
+ 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.
+
+ Solution O(n + m ^ 2): For every row, consider each pair of 1s (every column pairs) and sum up the previous
+ occurrence of 1s for the same column.
+
+ */
+public class CornerRectangles {
+
+    /**
+     * Main method
+     * @param args
+     * @throws Exception
+     */
+    public static void main(String[] args) throws Exception{
+        int[][] A = {{1, 1, 1}, {1, 1, 1}, {1, 1, 1}};
+        System.out.println(new CornerRectangles().countCornerRectangles(A));
+    }
+
+    public int countCornerRectangles(int[][] grid) {
+        int[][] count = new int[grid[0].length][grid[0].length];
+        int result = 0;
+        for(int[] row : grid){
+            for(int i = 0; i < row.length; i ++){
+                if(row[i] == 1){
+                    for(int j = i + 1; j < row.length; j++){
+                        if(row[j] == 1){
+                            if(count[i][j] > 0){
+                                result += count[i][j];
+                            }
+                            count[i][j] ++;
+                        }
+                    }
+                }
+            }
+        }
+        return result;
+    }
+
+}