LeetCode-Symmetric Tree

树的遍历，

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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if (root == NULL)
            return true;
        return symmetric(root->left, root->right);
    }
    bool symmetric(TreeNode *lnode, TreeNode *rnode) {
        if (lnode == NULL && rnode == NULL)
            return true;
        if (lnode == NULL || rnode == NULL) 
            return false;
        if (lnode->val != rnode->val) {
            return false;
        }
        return symmetric(lnode->left, rnode->right) && symmetric(lnode->right, rnode->left);
    }
};

迭代要用到队列；

/**
 * 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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if (root == NULL)
            return true;
        queue<TreeNode*> tqueue;
        tqueue.push(root->left);
        tqueue.push(root->right);
        while (!tqueue.empty()) {
            TreeNode *lnode = tqueue.front();
            tqueue.pop();
            TreeNode *rnode = tqueue.front();
            tqueue.pop();
            if (lnode == NULL && rnode == NULL)
                continue;
            if (lnode == NULL || rnode == NULL || (lnode->val != rnode->val))
                return false;
            tqueue.push(lnode->left);
            tqueue.push(rnode->right);
            tqueue.push(lnode->right);
            tqueue.push(rnode->left);
        }
        return true;
    }
};