101. Symmetric Tree_iterative.cc

//Using iterative (BFS-like) method
// TC: O(logn)
// SC: O(2^logn)
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    bool isSymmetric(TreeNode* root) {
        if(!root)
            return true;
        queue<TreeNode*> q;
        q.push(root->left);
        q.push(root->right);
        
        while (!q.empty()) {
            TreeNode* left = q.front();
            q.pop();
            TreeNode* right = q.front();
            q.pop();
            if (!left && !right) {
                continue;
            } else if (!left || !right) {
                return false;                
            } else if (left->val != right->val){
                return false;
            }
            q.push(left->left);
            q.push(right->right);
            q.push(left->right);
            q.push(right->left);
                    
        } 
        return true;
        
    }
};