Eric 的小站

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输入：root = [1,2,2,3,4,4,3]
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输出：true

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输入：root = [1,2,2,null,3,null,3]
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输出：false

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/**
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 * Definition for a binary tree node.
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 * struct TreeNode {
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 *     int val;
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 *     TreeNode *left;
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 *     TreeNode *right;
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 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
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 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
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 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
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 * };
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 */
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class Solution {
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public:
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    bool isSymmetric(TreeNode* root) {
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        return root == nullptr || isSy(root->left,root->right);
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    }
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private:
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    bool isSy(TreeNode* l,TreeNode*r){
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        if(l == nullptr && r==nullptr) return true;
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        if(l==nullptr || r==nullptr || l->val != r->val) return false;
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        return isSy(l->left,r->right) && isSy(l->right,r->left);
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    }
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};