From 82969c64098db6a0697f2aa16f9048c79aef4ac6 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=E5=94=90=E6=A0=91=E6=A3=AE?= <14021051@buaa.edu.cn>
Date: Wed, 31 Oct 2018 21:25:33 +0800
Subject: [PATCH] Create 101. Symmetric Tree.md

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+# [101. Symmetric Tree](https://leetcode.com/problems/symmetric-tree/description/)
+# 思路
+## 思路一: 递归
+最简单的思路就是递归。若一个非空树是对称树，那么其左右子树是互为镜像的。类似[判断两颗树是否相等的题解](https://github.com/ShusenTang/LeetCode/blob/master/100.%20Same%20Tree.md)，
+两棵非空树互为镜像的充要条件是:
+* 根的值相同;
+* 且树一的左子树和树二的右子树互为镜像，树一的右子树和树二的左子树互为镜像。
+## 思路二: 非递归
+采用类似层次遍历的方法。对于左子树从上往下**从左往右**层次遍历，对于右子树从上往下**从右往左**层次遍历，遍历过程中进行比较。
+
+# C++
+## 思路一
+``` C++
+class Solution {
+private:
+    bool isSymmetricSameTree(TreeNode* p, TreeNode* q) { // 判断两棵树是否互为镜像
+        if(p == NULL && q==NULL) return true;
+        if(p == NULL || q == NULL || p -> val != q -> val) return false;
+        return isSymmetricSameTree(p -> left, q -> right) && isSymmetricSameTree(p -> right, q -> left);
+    }
+public:
+    bool isSymmetric(TreeNode* root) {
+        if(root == NULL) return true;
+        return isSymmetricSameTree(root -> left, root -> right);
+    }
+};
+```
+## 思路二
+``` C++
+class Solution {
+public:
+bool isSymmetric(TreeNode *root) {
+        if (root == NULL) return true;
+        TreeNode *left, *right;
+
+        queue<TreeNode*> q1, q2;
+        q1.push(root->left);
+        q2.push(root->right);
+        while (!q1.empty() && !q2.empty()){
+            left = q1.front();
+            q1.pop();
+            right = q2.front();
+            q2.pop();
+            if (NULL == left && NULL == right)
+                continue;
+            if (NULL == left || NULL == right)
+                return false;
+            if (left->val != right->val)
+                return false;
+            q1.push(left->left);
+            q1.push(left->right);
+            q2.push(right->right);
+            q2.push(right->left);
+        }
+        return true;
+    }
+};
+```