Largest Rectangular Area in a Histogram using Stack
Last Updated :
11 Sep, 2023
Find the largest rectangular area possible in a given histogram where the largest rectangle can be made of a number of contiguous bars whose heights are given in an array. For simplicity, assume that all bars have the same width and the width is 1 unit.
Example:
Input: histogram = {6, 2, 5, 4, 5, 1, 6}
Output: 12
Input: histogram = {3, 5, 1, 7, 5, 9}
Output: 15
To solve the problem follow the below idea:
For every bar ‘x’, we calculate the area with ‘x’ as the smallest bar in the rectangle. If we calculate the such area for every bar ‘x’ and find the maximum of all areas, our task is done.
How to calculate the area with ‘x’ as the smallest bar?
We need to know the index of the first smaller (smaller than ‘x’) bar on the left of ‘x’ and the index of the first smaller bar on the right of ‘x’. Let us call these indexes as ‘left index’ and ‘right index’ respectively. We traverse all bars from left to right and maintain a stack of bars. Every bar is pushed to stack once. A bar is popped from the stack when a bar of smaller height is seen. When a bar is popped, we calculate the area with the popped bar as the smallest bar.
How do we get the left and right indexes of the popped bar?
The current index tells us the right index and the index of the previous item in the stack is the left index
Follow the given steps to solve the problem:
- Create an empty stack.
- Start from the first bar, and do the following for every bar hist[i] where ‘i‘ varies from 0 to n-1
- If the stack is empty or hist[i] is higher than the bar at top of the stack, then push ‘i‘ to stack.
- If this bar is smaller than the top of the stack, then keep removing the top of the stack while the top of the stack is greater.
- Let the removed bar be hist[tp]. Calculate the area of the rectangle with hist[tp] as the smallest bar.
- For hist[tp], the ‘left index’ is previous (previous to tp) item in stack and ‘right index’ is ‘i‘ (current index).
- If the stack is not empty, then one by one remove all bars from the stack and do step (2.2 and 2.3) for every removed bar
Below is the implementation of the above approach.
C++
#include <bits/stdc++.h>
using namespace std;
int getMaxArea( int hist[], int n)
{
stack< int > s;
int max_area = 0;
int tp;
int area_with_top;
int i = 0;
while (i < n) {
if (s.empty() || hist[s.top()] <= hist[i])
s.push(i++);
else {
tp = s.top();
s.pop();
area_with_top
= hist[tp]
* (s.empty() ? i : i - s.top() - 1);
if (max_area < area_with_top)
max_area = area_with_top;
}
}
while (s.empty() == false ) {
tp = s.top();
s.pop();
area_with_top
= hist[tp] * (s.empty() ? i : i - s.top() - 1);
if (max_area < area_with_top)
max_area = area_with_top;
}
return max_area;
}
int main()
{
int hist[] = { 6, 2, 5, 4, 5, 1, 6 };
int n = sizeof (hist) / sizeof (hist[0]);
cout << "Maximum area is " << getMaxArea(hist, n);
return 0;
}
|
Java
import java.util.Stack;
public class RectArea {
static int getMaxArea( int hist[], int n)
{
Stack<Integer> s = new Stack<>();
int max_area = 0 ;
int tp;
int area_with_top;
int i = 0 ;
while (i < n) {
if (s.empty() || hist[s.peek()] <= hist[i])
s.push(i++);
else {
tp = s.peek();
s.pop();
area_with_top
= hist[tp]
* (s.empty() ? i : i - s.peek() - 1 );
if (max_area < area_with_top)
max_area = area_with_top;
}
}
while (s.empty() == false ) {
tp = s.peek();
s.pop();
area_with_top
= hist[tp]
* (s.empty() ? i : i - s.peek() - 1 );
if (max_area < area_with_top)
max_area = area_with_top;
}
return max_area;
}
public static void main(String[] args)
{
int hist[] = { 6 , 2 , 5 , 4 , 5 , 1 , 6 };
System.out.println( "Maximum area is "
+ getMaxArea(hist, hist.length));
}
}
|
Python3
def max_area_histogram(histogram):
stack = list ()
max_area = 0
index = 0
while index < len (histogram):
if ( not stack) or (histogram[stack[ - 1 ]] < = histogram[index]):
stack.append(index)
index + = 1
else :
top_of_stack = stack.pop()
area = (histogram[top_of_stack] *
((index - stack[ - 1 ] - 1 )
if stack else index))
max_area = max (max_area, area)
while stack:
top_of_stack = stack.pop()
area = (histogram[top_of_stack] *
((index - stack[ - 1 ] - 1 )
if stack else index))
max_area = max (max_area, area)
return max_area
if __name__ = = '__main__' :
hist = [ 6 , 2 , 5 , 4 , 5 , 1 , 6 ]
print ( "Maximum area is" ,
max_area_histogram(hist))
|
C#
using System;
using System.Collections.Generic;
class GFG {
public static int getMaxArea( int [] hist, int n)
{
Stack< int > s = new Stack< int >();
int max_area = 0;
int tp;
int area_with_top;
int i = 0;
while (i < n) {
if (s.Count == 0 || hist[s.Peek()] <= hist[i]) {
s.Push(i++);
}
else {
tp = s.Peek();
s.Pop();
area_with_top
= hist[tp]
* (s.Count == 0 ? i
: i - s.Peek() - 1);
if (max_area < area_with_top) {
max_area = area_with_top;
}
}
}
while (s.Count > 0) {
tp = s.Peek();
s.Pop();
area_with_top
= hist[tp]
* (s.Count == 0 ? i : i - s.Peek() - 1);
if (max_area < area_with_top) {
max_area = area_with_top;
}
}
return max_area;
}
public static void Main( string [] args)
{
int [] hist = new int [] { 6, 2, 5, 4, 5, 1, 6 };
Console.WriteLine( "Maximum area is "
+ getMaxArea(hist, hist.Length));
}
}
|
Javascript
<script>
function max_area_histogram(histogram){
let stack = []
let max_area = 0
let index = 0
while (index < histogram.length){
if (stack.length == 0 || histogram[stack[stack.length-1]] <= histogram[index]){
stack.push(index)
index += 1
}
else {
let top_of_stack = stack.pop()
let area = (histogram[top_of_stack] *
(stack.length > 0 ? (index - stack[stack.length-1] - 1) : index))
max_area = Math.max(max_area, area)
}
}
while (stack.length > 0){
let top_of_stack = stack.pop()
let area = (histogram[top_of_stack] *
(stack.length > 0 ? (index - stack[stack.length-1] - 1) : index))
max_area = Math.max(max_area, area)
}
return max_area
}
let hist = [6, 2, 5, 4, 5, 1, 6]
document.write( "Maximum area is" , max_area_histogram(hist))
</script>
|
Output
Maximum area is 12
Time Complexity: O(N), Since every bar is pushed and popped only once
Auxiliary Space: O(N)
Largest Rectangular Area in a Histogram by finding the next and the previous smaller element:
To solve the problem follow the below idea:
Find the previous and the next smaller element for every element of the histogram, as this would help to calculate the length of the subarray in which this current element is the minimum element. So we can create a rectangle of size (current element * length of the subarray) using this element. Take the maximum of all such rectangles.
Follow the given steps to solve the problem:
- First, we will take two arrays left_smaller[] and right_smaller[] and initialize them with -1 and n respectively
- For every element, we will store the index of the previous smaller and next smaller element in left_smaller[] and right_smaller[] arrays respectively
- Now for every element, we will calculate the area by taking this ith element as the smallest in the range left_smaller[i] and right_smaller[i] and multiplying it with the difference of left_smaller[i] and right_smaller[i]
- We can find the maximum of all the areas calculated in step 3 to get the desired maximum area
Below is the implementation of the above approach.
C++
#include <bits/stdc++.h>
using namespace std;
int getMaxArea( int arr[], int n)
{
stack< int > s;
s.push(-1);
int area = arr[0];
int i = 0;
vector< int > left_smaller(n, -1), right_smaller(n, n);
while (i < n) {
while (!s.empty() && s.top() != -1
&& arr[s.top()] > arr[i]) {
right_smaller[s.top()] = i;
s.pop();
}
if (i > 0 && arr[i] == arr[i - 1]) {
left_smaller[i] = left_smaller[i - 1];
}
else {
left_smaller[i] = s.top();
}
s.push(i);
i++;
}
for ( int j = 0; j < n; j++) {
area = max(area, arr[j]
* (right_smaller[j]
- left_smaller[j] - 1));
}
return area;
}
int main()
{
int hist[] = { 6, 2, 5, 4, 5, 1, 6 };
int n = sizeof (hist) / sizeof (hist[0]);
cout << "maxArea = " << getMaxArea(hist, n) << endl;
return 0;
}
|
Java
import java.io.*;
import java.lang.*;
import java.util.*;
public class RectArea {
public static int getMaxArea( int arr[], int n)
{
Stack<Integer> s = new Stack<>();
s.push(- 1 );
int max_area = arr[ 0 ];
int left_smaller[] = new int [n];
int right_smaller[] = new int [n];
for ( int i = 0 ; i < n; i++) {
left_smaller[i] = - 1 ;
right_smaller[i] = n;
}
int i = 0 ;
while (i < n) {
while (!s.empty() && s.peek() != - 1
&& arr[i] < arr[s.peek()]) {
right_smaller[s.peek()] = ( int )i;
s.pop();
}
if (i > 0 && arr[i] == arr[(i - 1 )]) {
left_smaller[i]
= left_smaller[( int )(i - 1 )];
}
else {
left_smaller[i] = s.peek();
}
s.push(i);
i++;
}
for (i = 0 ; i < n; i++) {
max_area = Math.max(
max_area, arr[i]
* (right_smaller[i]
- left_smaller[i] - 1 ));
}
return max_area;
}
public static void main(String[] args)
{
int hist[] = { 6 , 2 , 5 , 4 , 5 , 1 , 6 };
System.out.println( "Maximum area is "
+ getMaxArea(hist, hist.length));
}
}
|
Python3
def getMaxArea(arr):
s = [ - 1 ]
n = len (arr)
area = 0
i = 0
left_smaller = [ - 1 ] * n
right_smaller = [n] * n
while i < n:
while s and (s[ - 1 ] ! = - 1 ) and (arr[s[ - 1 ]] > arr[i]):
right_smaller[s[ - 1 ]] = i
s.pop()
if ((i > 0 ) and (arr[i] = = arr[i - 1 ])):
left_smaller[i] = left_smaller[i - 1 ]
else :
left_smaller[i] = s[ - 1 ]
s.append(i)
i + = 1
for j in range ( 0 , n):
area = max (area, arr[j] * (right_smaller[j] - left_smaller[j] - 1 ))
return area
if __name__ = = '__main__' :
hist = [ 6 , 2 , 5 , 4 , 5 , 1 , 6 ]
print ( "maxArea = " , getMaxArea(hist))
|
C#
using System;
using System.Collections.Generic;
public class RectArea {
public static int getMaxArea( int [] arr, int n)
{
Stack< int > s = new Stack< int >();
s.Push(-1);
int max_area = arr[0];
int [] left_smaller = new int [n];
int [] right_smaller = new int [n];
for ( int j = 0; j < n; j++) {
left_smaller[j] = -1;
right_smaller[j] = n;
}
int i = 0;
while (i < n) {
while (s.Count != 0 && s.Peek() != -1
&& arr[i] < arr[s.Peek()]) {
right_smaller[s.Peek()] = ( int )i;
s.Pop();
}
if (i > 0 && arr[i] == arr[(i - 1)]) {
left_smaller[i]
= left_smaller[( int )(i - 1)];
}
else {
left_smaller[i] = s.Peek();
}
s.Push(i);
i++;
}
for (i = 0; i < n; i++) {
max_area = Math.Max(
max_area, arr[i]
* (right_smaller[i]
- left_smaller[i] - 1));
}
return max_area;
}
public static void Main(String[] args)
{
int [] hist = { 6, 2, 5, 4, 5, 1, 6 };
Console.WriteLine( "Maximum area is "
+ getMaxArea(hist, hist.Length));
}
}
|
Javascript
<script>
function getMaxArea(arr, n)
{
let s = [];
s.push(-1);
let area = arr[0];
let i = 0;
let left_smaller = new Array(n).fill(-1), right_smaller = new Array(n).fill(n);
while (i<n){
while (s.length > 0 &&s[s.length-1]!=-1&&arr[s[s.length-1]]>arr[i]){
right_smaller[s[s.length-1]] = i;
s.pop();
}
if (i>0&&arr[i]==arr[i-1]){
left_smaller[i] = left_smaller[i-1];
} else {
left_smaller[i] = s[s.length-1];
}
s.push(i);
i++;
}
for (let j = 0; j<n; j++){
area = Math.max(area, arr[j]*(right_smaller[j]-left_smaller[j]-1));
}
return area;
}
let hist = [6, 2, 5, 4, 5, 1, 6];
let n = hist.length;
document.write( "maxArea = " + getMaxArea(hist, n));
</script>
|
Time Complexity: O(N)
Auxiliary Space: O(N)
Related Articles: Divide and Conquer based O(N log N) solution
Thanks to Ashish Anand for suggesting initial solution.
Like Article
Suggest improvement
Share your thoughts in the comments
Please Login to comment...