Merge pull request #613 from ironartisan/master

添加0042.接雨水java双指针解法

problems/0042.接雨水.md

@@ -366,6 +366,58 @@ public:
 
 Java: 
 
+双指针法
+```java
+class Solution {
+    public int trap(int[] height) {
+        int sum = 0;
+        for (int i = 0; i < height.length; i++) {
+            // 第一个柱子和最后一个柱子不接雨水
+            if (i==0 || i== height.length - 1) continue;
+            
+            int rHeight = height[i]; // 记录右边柱子的最高高度
+            int lHeight = height[i]; // 记录左边柱子的最高高度
+            for (int r = i+1; r < height.length; r++) {
+                if (height[r] > rHeight) rHeight = height[r];
+            }
+            for (int l = i-1; l >= 0; l--) {
+                if(height[l] > lHeight) lHeight = height[l];
+            }
+            int h = Math.min(lHeight, rHeight) - height[i];
+            if (h > 0) sum += h;
+        }
+        return sum;
+
+    }
+}
+```
+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