【Leetcode】101. Symmetric Tree（二叉树是否为对称）

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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.

题目大意：

判断给出的二叉树是否是对称的。

解题思路：

同时判读左右两边的值，在递归过程中应该是判断（cor->l，cor->r）&&（cor->r，cor->l）。

/**
 * 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 {
private:
    bool valid(TreeNode* l, TreeNode *r){
        if(l == NULL && r == NULL){
            return true;
        }else if(l!=NULL && r !=NULL){
            
            if(l->val==r->val){
                return valid(l->left,r->right)&&valid(l->right, r->left);
            }else{
                return false;
            }
            
        }else{
            return false;
        }
    }
public:
    bool isSymmetric(TreeNode* root) {
        if(root==NULL){
            return true;
        }
        return valid(root->left, root->right);
    }
};