leetcode101.对称二叉树

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    bool compare(TreeNode* left,TreeNode* right){
        //左右节点都空为真，一空一有为假，都有则进一步判断
        if(left==NULL&&right==NULL)
            return true;
        else if(left==NULL&&right!=NULL)
            return false;
        else if(left!=NULL&&right==NULL)
            return false;
        else{
            //两者数值不等为假，两者数值相等则递归比较外侧和里侧
            if(left->val!=right->val)
                return false;
            else{
                bool outSide=compare(left->left,right->right);
                bool inSide=compare(left->right,right->left);
                return outSide&&inSide;
            }
        }
    }
    bool isSymmetric(TreeNode* root) {
        if(root==NULL)
            return true;
        else   
            return compare(root->left,root->right);
    }
};