leetcode#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
But the following [1,2,2,null,3,null,3] is not:

由于这道题data structure是二叉树，所以很容易就能想到递归地求解。所以我写了一个compareMirror的函数来判断其对称性。然后对于每一个节点地柜调用该函数即可。

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