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

Note:
Bonus points if you could solve it both recursively and iteratively.

解题思路：
类似100.Same Tree，不过将左右子树递归路径反向。

/** * 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 isSymmetric(TreeNode* root) {        if (root == NULL) return true;        return isSymmetric(root->left, root->right);    }    bool isSymmetric(TreeNode *left, TreeNode *right) {        if (!left && !right) return true;        if (!left || !right) return false;        return (left->val == right->val)            && isSymmetric(left->left, right->right)            && isSymmetric(left->right, right->left);    }};