[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 is symmetric:

    1
   / \
  2   2
 / \ / \
3  4 4  3

But the following is not:

    1
   / \
  2   2
   \   \
   3    3

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

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.

递归实现方法：

/**
 * 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 || (!root->left && !root->right)) {
            return true;
        } 
        
        bool rt = doJudge(root->left, root->right);
        return rt;
    }
    
    bool doJudge(TreeNode* lr, TreeNode *rr) {
        if ((lr == NULL && rr == NULL)) {
            return true;
        }
        if ((lr && !rr) || (!lr && rr) ) {
            return false;
        }
        if (lr->val != rr->val) {
            return false;
        }
        
        bool check1 = doJudge(lr->left, rr->right);
        bool check2 = doJudge(lr->right, rr->left);
        
        return (check1 && check2);
    }
};