From ac71ac43bcc9437839d0da2fc4615eca6b34a31e Mon Sep 17 00:00:00 2001
From: hailincai <hailin.cai.neulion@gmail.com>
Date: Sun, 5 Sep 2021 08:32:17 -0400
Subject: [PATCH] =?UTF-8?q?Update=200042.=E6=8E=A5=E9=9B=A8=E6=B0=B4.md?=
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增加Java 单调栈实现
---
 problems/0042.接雨水.md | 47 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 47 insertions(+)

diff --git a/problems/0042.接雨水.md b/problems/0042.接雨水.md
index 522b10f9..17c671ef 100644
--- a/problems/0042.接雨水.md
+++ b/problems/0042.接雨水.md
@@ -391,6 +391,7 @@ class Solution {
     }
 }
 ```
+
 动态规划法
 ```java
 class Solution {
@@ -418,6 +419,52 @@ class Solution {
     }
 }
 ```
+
+单调栈法
+```java
+    public int trapMonoStack(int[] height){
+        int size = height.length;
+
+        if (size <= 2) return 0;
+        
+        // in the stack, we push the index of array
+        // using height[] to access the real height
+        Stack<Integer> stack = new Stack<Integer>();
+        stack.push(0);
+
+        int sum = 0;
+        for (int index = 1; index < size; index++){
+            int stackTop = stack.peek();
+            if (height[index] < height[stackTop]){
+                stack.push(index);
+            }else if (height[index] == height[stackTop]){
+                // 因为相等的相邻墙，左边一个是不可能存放雨水的，所以pop左边的index, push当前的index
+                stack.pop();
+                stack.push(index);
+            }else{
+                //pop up all lower value
+                int heightAtIdx = height[index];
+                while (!stack.isEmpty() && (heightAtIdx > height[stackTop])){
+                    int mid = stack.pop();
+                    
+                    if (!stack.isEmpty()){
+                        int left = stack.peek();
+
+                        int h = Math.min(height[left], height[index]) - height[mid];
+                        int w = index - left - 1;
+                        int hold = h * w;
+                        if (hold > 0) sum += hold;
+                        stackTop = stack.peek();
+                    }
+                }
+                stack.push(index);
+            }
+        }
+        
+        return sum;
+    }
+```
+
 Python:
 
 双指针法