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

/** * Definition for a binary tree node. * 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;        else         return is(root->left, root->right);            }            bool is(TreeNode* left, TreeNode* right){                if(left == NULL || right == NULL)        return (left == right) ;                if(left->val != right->val)        return false;                return is(left->left, right->right) && is(left->right, right->left);                    }};

class Solution {public:    bool isSymmetric(TreeNode* root) {        TreeNode *left, *right;        if(!root)        return true;        queue<TreeNode*>q1, q2;        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;    }    };