【LeetCode】101. Symmetric Tree (2 solutions)

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:

But the following is not:

Note:
Bonus points if you could solve it both recursively and iteratively.

不管是递归还是非递归，找到关系就好做。

所谓对称，也就是：

1、left对应right

2、left->left对应right->right

3、left->right对应right->left

解法一：递归

/**

 * Definition for binary tree

 * struct TreeNode {

 *     int val;

 *     TreeNode *left;

 *     TreeNode *right;

 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}

 * };

 */

class Solution {

public:

    bool isSymmetric(TreeNode *root) {

        if(!root)

            return true;

        return isSymTree(root->left, root->right);

    }

    bool isSymTree(TreeNode* p, TreeNode* q)

    {

        if(!isSameNode(p, q))

            return false;

        if(!p && !q)

            return true;

        return isSymTree(p->left, q->right) && isSymTree(p->right, q->left);

    }

    bool isSameNode(TreeNode* p, TreeNode* q)

    {

        if(!p && !q)

            return true;

        if((!p && q) || (p && !q) || (p->val != q->val))

            return false;

        return true;

    }

};

解法二：非递归

使用两个队列，对左右子树分别进行层次遍历。

进队时的对应元素比较即可。

/**

 * Definition for binary tree

 * struct TreeNode {

 *     int val;

 *     TreeNode *left;

 *     TreeNode *right;

 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}

 * };

 */

class Solution {

public:

    bool isSymmetric(TreeNode *root) {

        if(!root)

            return true;

        if(!isSameNode(root->left, root->right))

            return false;

        if(!root->left && !root->right)

            return true;

        queue<TreeNode*> lqueue;

        queue<TreeNode*> rqueue;

        lqueue.push(root->left);

        rqueue.push(root->right);

        while(!lqueue.empty() && !rqueue.empty())

        {

            TreeNode* lfront = lqueue.front();

            TreeNode* rfront = rqueue.front();

            lqueue.pop();

            rqueue.pop();

            if(!isSameNode(lfront->left, rfront->right))

                return false;

            if(lfront->left && rfront->right)

            {

                lqueue.push(lfront->left);

                rqueue.push(rfront->right);

            }

            if(!isSameNode(lfront->right, rfront->left))

                return false;

            if(lfront->right && rfront->left)

            {

                lqueue.push(lfront->right);

                rqueue.push(rfront->left);

            }

        }

        return true;

    }

    bool isSameNode(TreeNode* p, TreeNode* q)

    {

        if(!p && !q)

            return true;

        if((!p && q) || (p && !q) || (p->val != q->val))

            return false;

        return true;

    }

};

巴特西

【LeetCode】101. Symmetric Tree (2 solutions)

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