Moved StackPostfixNotation.java from the Others section to the Stack section (#4372)

* Moved StackPostfixNotation.java from the Others section to the Stack section

* Put all stack related algo in a separate stack directory in the algorithms directory. The stack directory under data-structures now only contains various implementations of the stack data structure.

* formatted files

src/main/java/com/thealgorithms/stacks/LargestRectangle.java

@@ -0,0 +1,35 @@
+package com.thealgorithms.stacks;
+
+import java.util.Stack;
+
+/**
+ *
+ * @author mohd rameez github.com/rameez471
+ */
+
+public class LargestRectangle {
+
+    public static String largestRectanglehistogram(int[] heights) {
+        int n = heights.length, maxArea = 0;
+        Stack<int[]> st = new Stack<>();
+        for (int i = 0; i < n; i++) {
+            int start = i;
+            while (!st.isEmpty() && st.peek()[1] > heights[i]) {
+                int[] tmp = st.pop();
+                maxArea = Math.max(maxArea, tmp[1] * (i - tmp[0]));
+                start = tmp[0];
+            }
+            st.push(new int[] {start, heights[i]});
+        }
+        while (!st.isEmpty()) {
+            int[] tmp = st.pop();
+            maxArea = Math.max(maxArea, tmp[1] * (n - tmp[0]));
+        }
+        return Integer.toString(maxArea);
+    }
+
+    public static void main(String[] args) {
+        assert largestRectanglehistogram(new int[] {2, 1, 5, 6, 2, 3}).equals("10");
+        assert largestRectanglehistogram(new int[] {2, 4}).equals("4");
+    }
+}