101.symmetric-tree.cpp

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
// version 1: recursive version
/*
class Solution {
public:
    bool isSymmetric(TreeNode* root) {
        if (!root) {
            return true;
        }
        return helper(root->left, root->right);
    }
    bool helper(TreeNode* left, TreeNode* right) {
        if (!left && !right) {
            return true;
        }
        if ((left && !right) || (right && !left) || (left->val != right->val)) {
            return false;
        }
        return helper(left->right, right->left) && helper(left->left, right->right);
    }
};
*/
class Solution {
public:
    bool isSymmetric(TreeNode* root) {
        if (!root) {
            return true;
        }
        queue<TreeNode*> q1, q2;
        q1.push(root->left);
        q2.push(root->right);
        while (!q1.empty()) {
            TreeNode *node1 = q1.front(); q1.pop();
            TreeNode *node2 = q2.front(); q2.pop();
            if (!node1 && !node2) {
                continue;
            }
            if ((node1 && !node2) || (node2 && !node1) || (node2->val != node1->val)) {
                return false;
            }
            q1.push(node1->left);
            q1.push(node1->right);
            q2.push(node2->right);
            q2.push(node2->left);
        }
        return true;
    }
};