【leetcode】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

 

思路：判断树是否是对称的 递归判断即可

class Solution {

public:

    bool isSymmetric(TreeNode* root) {

        if(NULL == root)   return true;



        TreeNode * L = root->left;

        TreeNode * R = root->right;

        return isSymmetricPart(L, R);

    }



     bool isSymmetricPart(TreeNode* L, TreeNode* R)

     {

         if(NULL == L && NULL == R) return true;

         if(NULL == L && NULL != R || NULL == R && NULL != L) return false;

         if(L->val == R->val)

             return isSymmetricPart(L->left, R->right) && isSymmetricPart(L->right, R->left);

         else

             return false;

         

     }

};

 

更简短的写法

class Solution {

public:

    bool isSymmetric(TreeNode* root) {

        return !root ? true : isSymmetricHelper(root->left, root->right);

    }



    bool isSymmetricHelper(TreeNode* left, TreeNode* right) {

        if (!left && !right) { return true; }

        return 

            (left && right) &&

            (left->val == right->val) &&

            isSymmetricHelper(left->left, right->right) &&

            isSymmetricHelper(left->right, right->left);

    }

};