Merge pull request #97 from Joshua-Lu/patch-13

添加 0222.完全二叉树的节点个数 Java版本，2种解法

problems/0222.完全二叉树的节点个数.md

@@ -196,13 +196,44 @@ public:
 Java：
 ```java
 class Solution {
+    // 通用递归解法
     public int countNodes(TreeNode root) {
-        if (root == null) return 0;
+        if(root == null) {
+            return 0;
+        }
+        return countNodes(root.left) + countNodes(root.right) + 1;
+    }
+}
+```
 
-        int leftnum = countNodes(root.left);
-        int rightnum = countNodes(root.right);
+```java
+class Solution {
+    /**
+     * 针对完全二叉树的解法
+     *
+     * 满二叉树的结点数为：2^depth - 1
+     */
+    public int countNodes(TreeNode root) {
+        if(root == null) {
+            return 0;
+        }
+        int leftDepth = getDepth(root.left);
+        int rightDepth = getDepth(root.right);
+        if (leftDepth == rightDepth) {// 左子树是满二叉树
+            // 2^leftDepth其实是 （2^leftDepth - 1） + 1 ，左子树 + 根结点
+            return (1 << leftDepth) + countNodes(root.right);
+        } else {// 右子树是满二叉树
+            return (1 << rightDepth) + countNodes(root.left);
+        }
+    }
 
-        return leftnum + rightnum + 1;
+    private int getDepth(TreeNode root) {
+        int depth = 0;
+        while (root != null) {
+            root = root.left;
+            depth++;
+        }
+        return depth;
     }
 }
 ```