添加0042.接雨水java动态规划解法

problems/0042.接雨水.md

@@ -365,6 +365,7 @@ public:
 ## 其他语言版本 
 
 Java: 
+
 双指针法
 ```java
 class Solution {
@@ -390,6 +391,33 @@ class Solution {
     }
 }
 ```
+动态规划法
+```java
+class Solution {
+    public int trap(int[] height) {
+        int length = height.length;
+        if (length <= 2) return 0;
+        int[] maxLeft = new int[length];
+        int[] maxRight = new int[length];
+        
+        // 记录每个柱子左边柱子最大高度
+        maxLeft[0] = height[0];
+        for (int i = 1; i< length; i++) maxLeft[i] = Math.max(height[i], maxLeft[i-1]);
+        
+        // 记录每个柱子右边柱子最大高度
+        maxRight[length - 1] = height[length - 1];
+        for(int i = length - 2; i >= 0; i--) maxRight[i] = Math.max(height[i], maxRight[i+1]);
+        
+        // 求和
+        int sum = 0;
+        for (int i = 0; i < length; i++) {
+            int count = Math.min(maxLeft[i], maxRight[i]) - height[i];
+            if (count > 0) sum += count;
+        }
+        return sum;
+    }
+}
+```
 Python:
 
 双指针法