From 8d1c4d65ffaf633ac3ffe17430bee6d7e5759829 Mon Sep 17 00:00:00 2001
From: hailincai <hailin.cai.neulion@gmail.com>
Date: Sun, 5 Sep 2021 09:17:11 -0400
Subject: [PATCH] =?UTF-8?q?Update=200084.=E6=9F=B1=E7=8A=B6=E5=9B=BE?=
 =?UTF-8?q?=E4=B8=AD=E6=9C=80=E5=A4=A7=E7=9A=84=E7=9F=A9=E5=BD=A2.md?=
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增加java 单调栈实现
---
 problems/0084.柱状图中最大的矩形.md | 44 ++++++++++++++++++++
 1 file changed, 44 insertions(+)

diff --git a/problems/0084.柱状图中最大的矩形.md b/problems/0084.柱状图中最大的矩形.md
index a0f06e8f..57df4161 100644
--- a/problems/0084.柱状图中最大的矩形.md
+++ b/problems/0084.柱状图中最大的矩形.md
@@ -228,6 +228,50 @@ class Solution {
 }
 ```
 
+单调栈
+```java
+class Solution {
+    int largestRectangleArea(int[] heights) {
+        Stack<Integer> st = new Stack<Integer>();
+        
+        // 数组扩容，在头和尾各加入一个元素
+        int [] newHeights = new int[heights.length + 2];
+        newHeights[0] = 0;
+        newHeights[newHeights.length - 1] = 0;
+        for (int index = 0; index < heights.length; index++){
+            newHeights[index + 1] = heights[index];
+        }
+
+        heights = newHeights;
+        
+        st.push(0);
+        int result = 0;
+        // 第一个元素已经入栈，从下表1开始
+        for (int i = 1; i < heights.length; i++) {
+            // 注意heights[i] 是和heights[st.top()] 比较 ，st.top()是下表
+            if (heights[i] > heights[st.peek()]) {
+                st.push(i);
+            } else if (heights[i] == heights[st.peek()]) {
+                st.pop(); // 这个可以加，可以不加，效果一样，思路不同
+                st.push(i);
+            } else {
+                while (heights[i] < heights[st.peek()]) { // 注意是while
+                    int mid = st.peek();
+                    st.pop();
+                    int left = st.peek();
+                    int right = i;
+                    int w = right - left - 1;
+                    int h = heights[mid];
+                    result = Math.max(result, w * h);
+                }
+                st.push(i);
+            }
+        }
+        return result;
+    }
+}
+```
+
 Python:
 
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