Update 0042.接雨水.md

增加Java 单调栈实现

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