How to Check If Two Binary Trees are the Same?

/**
 * 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 isSameTree(TreeNode *p, TreeNode *q) {
        // empty nodes are equal
        if ((p == NULL) && (q == NULL)) return true; 
        // structure not equal (one of them is empty)
        if (p == NULL) return false;  
        if (q == NULL) return false;
        // value must be the same and two sub trees must be the same
        return (p->val == q->val) && isSameTree(p->left, q->left) && isSameTree(p->right, q->right);
    }
};

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} p
 * @param {TreeNode} q
 * @return {boolean}
 */
var isSameTree = function(p, q) {
    if (p === null && q === null) return true;
    if (p === null || q === null) return false;
    return (p.val === q.val) && isSameTree(p.left, q.left) && isSameTree(p.right, q.right);
};

/**
 * 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 isSameTree(TreeNode* p, TreeNode* q) {
        stack<pair<TreeNode*, TreeNode*>> st;
        st.push({p, q});
        while (!st.empty()) {
            auto p = st.top();
            st.pop();
            auto left = p.first;
            auto right = p.second;
            if (left && right) {
                if (left->val != right->val) {
                    return false;
                }                
                st.push({left->left, right->left});
                st.push({left->right, right->right});
            } else {
                if ((left != nullptr) ^ (right != nullptr)) {
                    return false;
                }
            }
        }
        return true;
    }
};