add java solution for leetcode 101

Author: Zhanganjin0520

README.md

@@ -311,7 +311,7 @@ LeetCode
 |104|[Maximum Depth of Binary Tree](https://leetcode.com/problems/maximum-depth-of-binary-tree/)| |Easy|
 |103|[Binary Tree Zigzag Level Order Traversal](https://leetcode.com/problems/binary-tree-zigzag-level-order-traversal/)| |Medium|
 |102|[Binary Tree Level Order Traversal](https://leetcode.com/problems/binary-tree-level-order-traversal/)| |Easy|
-|101|[Symmetric Tree](https://leetcode.com/problems/symmetric-tree/)| |Easy|
+|101|[Symmetric Tree](https://leetcode.com/problems/symmetric-tree/)| [java](./algorithms/sysmetricTree/Solution.java) |Easy|
 |100|[Same Tree](https://leetcode.com/problems/same-tree/)| |Easy|
 |99|[Recover Binary Search Tree](https://leetcode.com/problems/recover-binary-search-tree/)| |Hard|
 |98|[Validate Binary Search Tree](https://leetcode.com/problems/validate-binary-search-tree/)| |Medium|

algorithms/sysmetricTree/Solution.java

@@ -0,0 +1,17 @@
+public class Solution {
+    //从上向下递归，检验镜像树
+    public boolean isSymmetric(TreeNode root) {
+        if(null == root){
+            return true;
+        }
+        return isMirror(root.left, root.right);
+    }
+
+    private boolean isMirror(TreeNode left, TreeNode right){
+        if(left == null && right == null) return true;
+        if(left == null || right == null) return false;
+        return (left.val == right.val) 
+        && isMirror(left.left, right.right) 
+        && isMirror(left.right, right.left) ;
+    }
+}