Last Updated: 20 Nov, 2020
Maximum Size Rectangle Sub-matrix With All 1's
Hard
Asked in companies
You are given an 'N' * 'M' sized binary-valued matrix 'MAT, where 'N' is the number of rows and 'M' is the number of columns. You need to return the maximum size (area) of the submatrix which consists of all 1’s i.e. the maximum area of a submatrix in which each cell has only the value ‘1’.
In the above image, areas in green, red, and violet color are all submatrices of the original 4x4 matrix.
Note:
1. Binary valued matrix has only two values in each cell : 0 and 1.
2. A submatrix is a matrix formed by selecting certain rows and columns from a larger matrix.
3. The area of a matrix with 'h' rows and 'w' columns is equal to 'h' * 'w'.
Input Format:
The first line of the input contains an integer 'T' denoting the number of test cases.
The first line of each test case contains two space-separated integers 'N' and 'M', where 'N' = the number of rows in the given matrix and 'M' = the number of columns in the given matrix.
Then 'N' lines follow for each test case. Each line contains 'M' space-separated integers (either 1 or 0) denoting matrix elements.
Output Format:
For each test case print in a single line the area of maximum size submatrix of all 1’s in the given matrix on a new line.
The output of each test case will be printed in a separate line.
Note:
You do not need to print anything, it has already been taken care of. Just implement the given function.
Constraints:
1 <= 'T' <= 50
1 <= 'N', 'M' <= 100
Time Limit: 1 sec
Approaches
- We start from the first row and move downwards.
- We create three 1-dimensional arrays HEIGHT[], LEFT[], RIGHT[].
- ‘HEIGHT’[i]: stores the number of current continuous 1’s in column i.
- LEFT[i] : stores the leftmost index ‘j’ such that all indices say ‘K’, ‘K’ belongs to [j, i], ‘HEIGHT’[k] >= ‘HEIGHT’[i].
- RIGHT [i]: stores the rightmost index ‘j’ such that all indices say ‘K’, ‘K’ belongs to [i, j], ‘HEIGHT’[k] >= ‘HEIGHT’[i].
- By the above definitions, it’s easier to figure out that we can update our maximum area with the value (‘HEIGHT’[i] * (RIGHT [i] - LEFT [i])).
- Now let’s think how will we update the above three 1-dimensional arrays itself.
- Initialize the ‘HEIGHT’[] array with 0.
- Iterate through the rows from 0 to N-1.
- For each column:
- If the current value (MAT [i][j]) is 1, then update ‘HEIGHT’[j]++.
- Else reset the value of ‘HEIGHT’[j] to 0.
- Note that:
- Initially, ‘HEIGHT’ was initialized to 0. So LEFT[] array should be initialised to 0 as per definition.
- And similarly, RIGHT[] array should be initialized to M.
- Updating the LEFT[] array in each iteration among rows 0 to N-1:
- We scan from left to right.
- Initialize LEFTBOUNDARY = 0, which means left boundary for all 1’s in the current row.
- Iterate in the current row:
- If the current value (MAT [i][j]) is 1, then you can reset the LEFT[j] from 0 to LEFTBOUNDARY.
- Else left[j] = 0 and update LEFTBOUNDARY to j+1. As the current value is 0, so next LEFTBOUNDARY for all the cells in the Matrix ahead of column j is at least j+1.
- Similarly, for the RIGHT[] array in each iteration among rows 0 to N-1:
- We scan from right to left.
- Initialize RIGHTBOUNDARY = 0, which means right boundary for all 1’s in the current row.
- Iterate in the current row:
- If the current value (MAT[i][j]) is 1, then you can reset the RIGHT[j] from M to RIGHTBOUNDARY.
- Else RIGHT[j] = 0 and update RIGHTBOUNDARY to j-1. As current value is 0, so next RIGHTBOUNDARY for all the cells in the Matrix before column j is atmost j-1.
- Let’s first understand how do we calculate the largest area in a histogram.
- Let’s take an example image:
In the above example, the largest rectangle area in the histogram is 5*2 = 10 units.
- The trick is to maintain a data structure that could mimic the function of the monotone increasing queue, which a stack can perform.
- Suppose you have an input array HEIGHT[0], HEIGHT[1], HEIGHT[2] …. HEIGHT[N-1].
- Let’s suppose that we are at some index ‘J’, then we try to fix HEIGHT[J] as the right side of the rectangle, then any of the heights { HEIGHT[0] …. HEIGHT[J-1] } in the monotone increasing queue can be the left side of the rectangle.
- Before adding current height, i.e. HEIGHT[J], to monotone increasing queue, for all the values in the queue that have HEIGHT[I] >= HEIGHT[J] can be part of a rectangle of fixed height, HEIGHT[J]. Thus, all such HEIGHT[I] is possible left sides of the current rectangle. Maximize the area of the rectangle possible in the histogram.
- Pop all such heights from the queue and increase the breadth of the rectangle by 1. When there are no more elements to pop out of the queue. Push the current index in the queue, as it is guaranteed to follow monotone increasing height property.
- Note that, if the histogram contains just one height, for such cases, initially push 0 to the queue to calculate maximum area.
- At each row, we will maintain an additional 1- dimensional array HEIGHT[] which denotes the height of contiguous 1’s at any column ‘J’, 0 <= ‘J’ <= 'M'-1, from bottom to top. Here bottom means current row ‘I’.
- If the value of MAT[I][J] = 0, then HEIGHT[J] is set to 0, else it is incremented by 1.
- This 1-dimensional height array at each row mimics the histogram. The only thing left is to apply the “Largest area in histogram” function to the updated HEIGHT[] array in each row.
- Maximize area at each step and return.