added Maximum area rectangle in histogram problem

Author: sumityadav29

Practice Problems/Stack/Maximum area reactangle in histogram/MaximumArea.java

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+import java.util.*;
+import java.io.*;
+
+public class MaximumArea {
+    
+    public static void main(String[] args){
+        int hist[] = {2, 1, 4, 5, 1, 3, 3};
+        int maximumArea = largestRectangleArea(hist);
+        System.out.println(maximumArea);
+    }
+    
+    public static int largestRectangleArea(int[] A) {
+        Stack<Integer> st = new Stack<Integer>();
+        int ans = 0;
+        for (int i = 0 ; i < A.length || !st.isEmpty(); ){
+            if (i >= A.length){
+                ans = Math.max(ans, A[st.pop()] * (st.isEmpty() ? i : i - st.peek() - 1) );
+                continue;
+            }
+            if (st.isEmpty() || A[st.peek()] <= A[i]){
+                st.push(i++);
+            }else if (A[st.peek()] > A[i]){
+                ans = Math.max(ans, A[st.pop()] * (st.isEmpty() ? i : i - st.peek() - 1) );
+            }
+        }
+        return ans;
+    }
+}
+

Practice Problems/Stack/Maximum area reactangle in histogram/README.md

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+# Largest Rectangle in a Histogram
+
+** 2003/2004 ACM ICPC University of Ulm Local Contest **
+
+A histogram is a polygon composed of a sequence of rectangles aligned at a common base line. The rectangles have equal widths but may have different heights.Usually, histograms are used to represent discrete distributions, e.g., the frequencies of characters in texts. Note that the order of the rectangles, i.e., their heights, is important. Given, heights of rectangles, calculate the area of the largest rectangle in a histogram that is aligned at the common base line, too.
+
+### Example:
+**Input** : `[2, 1, 4, 5, 1, 3, 3]`
+
+**Output:** : `8`