链接

我的错误代码：

class Solution {
public:

    bool isSymmetric(TreeNode* root) {
        if(!root)return true;
        if(!root->left&&!root->right)return true;
        if(!(root->left&&root->right))return false;
        if(root->left->val!=root->right->val)return false;
        return isSymmetric(root->left)&&isSymmetric(root->right);
    }
};




class Solution {
public:
    bool isSymmetric(TreeNode* root) {
        if(!root)return true;
        return isMirror(root->left,root->right);
    }
    bool isMirror(TreeNode*left,TreeNode*right){
        if(!left&&!right)return true;
        if(!left||!right)return false;
        return (left->val==right->val)&&
        isMirror(left->left,left->right)&&
        isMirror(right->left,right->right);
    }
};

正确代码：

class Solution {
public:
    bool isSymmetric(TreeNode* root) {
        if(!root)return true;
        return isMirror(root->left,root->right);
    }
    bool isMirror(TreeNode*left,TreeNode*right){
        if(!left&&!right)return true;
        if(!left||!right)return false;
        return (left->val==right->val)&&
        isMirror(left->left,right->right)&&
        isMirror(right->left,left->right);
    }
};

或者：

class Solution {
public:
    bool isSymmetric(TreeNode* root) {
        if(!root)return true;
        return dfs(root->left,root->right);
    }
    bool dfs(TreeNode* p,TreeNode*q){
        if(!p&&!q)return true;
        if(!p||!q||p->val!=q->val)return false;
        return dfs(p->left,q->right)&&dfs(p->right,q->left);
    }
};

唯一注意的点：镜像是左孩子的左子树==右孩子的右子树且左孩子的右子树==右孩子的左子树