coder-chenhao
diff --git a/‎高频面试系列/接雨水.md
Lines changed: 83 additions & 1 deletion b/‎高频面试系列/接雨水.md
Lines changed: 83 additions & 1 deletion
@@ -183,6 +183,88 @@ if (l_max < r_max) {
 
 ![labuladong](../pictures/labuladong.jpg)
 
+暴力解法
+
+```java
+public int trap(int[] height) {
+    int ans = 0;
+    for (int i = 1; i < height.length - 1; i++) {
+        int leftMax = 0, rightMax = 0;
+        // 找右边最高的柱子
+        for (int j = i; j < height.length; j++) {
+            rightMax = Math.max(height[j], rightMax);
+        }
+        // 找左边最高的柱子
+        for (int j = i; j >= 0; j--) {
+            leftMax = Math.max(height[j], leftMax);
+        }
+        // 如果自己就是最高的话，
+        // leftMax == rightMax == height[i]
+        ans += Math.min(leftMax, rightMax) - height[i];
+    }
+    return ans;
+}
+```
+
+备忘录优化解法
+
+```java
+public int trap(int[] height) {
+    if (height == null || height.length == 0) return 0;
+    int ans = 0;
+    // 数组充当备忘录
+    int[] leftMax = new int[height.length];
+    int[] rightMax = new int[height.length];
+    // 初始化base case
+    leftMax[0] = height[0];
+    rightMax[height.length - 1] = height[height.length - 1];
+    // 从左到右计算leftMax
+    for (int i = 1; i < height.length; i++) {
+        leftMax[i] = Math.max(height[i], leftMax[i-1]);
+    }
+    // 从右到左计算rightMax
+    for (int i = height.length - 2; i >= 0; i--) {
+        rightMax[i] = Math.max(height[i], rightMax[i + 1]);
+    }
+    // 计算结果
+    for (int i = 1; i < height.length - 1; i++) {
+        ans += Math.min(leftMax[i], rightMax[i]) - height[i];
+    }
+    return ans;
+}
+```
+
+双指针解法
+
+```java
+public int trap(int[] height) {
+    if (height == null || height.length == 0) return 0;
+
+     int ans = 0;
+     int leftMax, rightMax;
+     // 左右指针
+     int left = 0, right = height.length - 1;
+     // 初始化
+     leftMax = height[0];
+     rightMax = height[height.length - 1];
+
+     while (left < right) {
+         // 更新左右两边柱子最大值
+         leftMax = Math.max(height[left], leftMax);
+         rightMax = Math.max(height[right], rightMax);
+
+         // 相当于ans += Math.min(leftMax, rightMax) - height[i]
+         if (leftMax < rightMax) {
+             ans += leftMax - height[left];
+             left++;
+         } else  {
+             ans += rightMax - height[right];
+             right--;
+         }
+     }
+     return ans;
+}
+```
 
 [eric wang](https://www.github.com/eric496) 提供 Java 代码
 
@@ -248,4 +330,4 @@ def trap(self, height: List[int]) -> int:
 
 [下一篇：如何去除有序数组的重复元素](../高频面试系列/如何去除有序数组的重复元素.md)
 
-[目录](../README.md#目录)
+[目录](../README.md#目录)