1 /** 2  * Definition for binary tree 3  * struct TreeNode { 4  *     int val; 5  *     TreeNode *left; 6  *     TreeNode *right; 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} 8  * }; 9  */10 class Solution {11 public:12     bool isSymmetric(TreeNode *root) {13         vector<int> inorder;14         stack<TreeNode*> s;15         TreeNode *cur = root;16         while(cur != NULL || !s.empty()){17             while(cur != NULL){18                 s.push(cur);19                 cur = cur->left;20             }21             cur = s.top();22             inorder.push_back(cur->val);23             s.pop();24             cur = cur->right;25         }26         if(inorder.size()%2 != 0){27             int l = 0;28             int r = inorder.size()-1;29             while(l < r){30                 if(inorder[l] != inorder[r]){31                     return false;32                 }33             }34             return true;35         }36         else{37             return false;38         }39     }40 };

 1 /** 2  * Definition for binary tree 3  * struct TreeNode { 4  *     int val; 5  *     TreeNode *left; 6  *     TreeNode *right; 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} 8  * }; 9  */10 class Solution {11 public:12     bool isSymmetric(TreeNode *root) {13         if(root == NULL)14             return true;15         return Recursive(root->left,root->right);16     }17     bool Recursive(TreeNode *a,TreeNode *b){18         if(a==NULL && b==NULL)return true;19         if(!a || !b)return false;20         if(a->val != b->val)return false;21         return Recursive(a->left,b->right)&&Recursive(a->right,b->left);22     }23 };

 1 /** 2  * Definition for binary tree 3  * struct TreeNode { 4  *     int val; 5  *     TreeNode *left; 6  *     TreeNode *right; 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} 8  * }; 9  */10 class Solution {11 public:12     bool isSymmetric(TreeNode *root) {13         if(root == NULL)14             return true;15         return Iterative(root);16     }17     bool Iterative(TreeNode *root){18         stack<TreeNode*> s1;19         stack<TreeNode*> s2;20         s1.push(root->left);21         s2.push(root->right);22         while(!s1.empty() && !s2.empty()){23             TreeNode *tmp1 = s1.top();24             s1.pop();25             TreeNode *tmp2 = s2.top();26             s2.pop();27             if((tmp1==NULL && tmp2!=NULL)||(tmp1!=NULL && tmp2==NULL))28                 return false;29             if(tmp1!=NULL && tmp2!=NULL){30                 if(tmp1->val!=tmp2->val)return false;31                 s1.push(tmp1->left);32                 s1.push(tmp1->right);33                 s2.push(tmp2->right);34                 s2.push(tmp2->left);35             }36         }37         return true;38     }39 };