Update 0084.柱状图中最大的矩形.md

添加单调栈精简java代码

problems/0084.柱状图中最大的矩形.md

@@ -307,6 +307,33 @@ class Solution {
     }
 }
 ```
+单调栈精简
+```java
+class Solution {
+    public int largestRectangleArea(int[] heights) {
+        int[] newHeight = new int[heights.length + 2];
+        System.arraycopy(heights, 0, newHeight, 1, heights.length);
+        newHeight[heights.length+1] = 0;
+        newHeight[0] = 0;
+
+        Stack<Integer> stack = new Stack<>();
+        stack.push(0);
+
+        int res = 0;
+        for (int i = 1; i < newHeight.length; i++) {
+            while (newHeight[i] < newHeight[stack.peek()]) {
+                int mid = stack.pop();
+                int w = i - stack.peek() - 1;
+                int h = newHeight[mid];
+                res = Math.max(res, w * h);
+            }
+            stack.push(i);
+
+        }
+        return res;
+    }
+}
+```
 
 Python3: