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Problem

Given an array of integers heights representing the histogram’s bar height where the width of each bar is 1, return the area of the largest rectangle in the histogram.

Example 1:

img

Input: heights = [2,1,5,6,2,3]
Output: 10
Explanation: The above is a histogram where width of each bar is 1.
The largest rectangle is shown in the red area, which has an area = 10 units.

Example 2:

img

Input: heights = [2,4]
Output: 4

Code

class Solution {
    public int largestRectangleArea(int[] heights) {
        if(heights == null || heights.length == 0) return 0;
        
        Stack<Integer> stack = new Stack<>();
        int maxArea = 0;
        
        for(int i = 0; i <= heights.length; i++){
            int currHeight = i == heights.length ? 0 : heights[i];
            
            while(!stack.isEmpty() && currHeight <= heights[stack.peek()]){
                int tempHeight = heights[stack.pop()];
                int start = stack.isEmpty() ? -1 : stack.peek();
                int area = tempHeight * (i - start - 1);
                maxArea = Math.max(maxArea, area);
            }
            stack.push(i);
        }
        
        return maxArea;
    }
}