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101. Symmetric Tree

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree [1,2,2,3,4,4,3] is symmetric:

    1   / \  2   2 / \ / \3  4 4  3

But the following [1,2,2,null,3,null,3] is not:

    1   / \  2   2   \   \   3    3

Solution

8 ms 16.9 MB

class Solution {   public:    bool isSymmetric(TreeNode* root) {           if(root == NULL) return true;        queue<TreeNode*> q1,q2;        TreeNode *left, *right;        q1.push(root->left);        q2.push(root->right);        while(!q1.empty() && !q2.empty())        {               left = q1.front();q1.pop();            right = q2.front();q2.pop();            if(left == NULL && right == NULL) continue;            if(left == NULL || right == NULL) return false;            if(left->val != right->val) return false;            q1.push(left->left);            q1.push(left->right);            q2.push(right->right);            q2.push(right->left);        }        return true;    }};

使用queue，按顺序将节点入队：

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