101. Symmetric Tree

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree [1,2,2,3,4,4,3] is symmetric:

1

/ \
2 2
/ \ / \
3 4 4 3
But the following [1,2,2,null,3,null,3] is not:
1
/ \
2 2
\ \
3 3
这道题采用BFS算法，一次遍历同一层次的结点，并且比较对称位置的val是否相同。代码如下：

/** * Definition for a binary tree node. * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    bool testSymmetric(TreeNode *left,TreeNode *right){        if(left==NULL&&right==NULL)return true;        else if(left!=NULL&&right==NULL)return false;        else if(left==NULL && right!=NULL)return false;        else if(left!=NULL &&right!=NULL&&right->val!=left->val)return false;        else if(left!=NULL&&right!=NULL&&right->val==left->val){            return testSymmetric(left->left,right->right)&&testSymmetric(left->right,right->left);        }    }    bool isSymmetric(TreeNode* root) {        if(root==NULL)return true;        else{            return testSymmetric(root->left,root->right);        }    }};