// 演算法：與 C 版相同，遞迴同時比較兩棵樹。
// Algorithm: same as the C version — recursively compare both trees together (DFS).
// Same = roots equal AND left subtrees same AND right subtrees same.

/**
 * 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) {
        // 邊界1：兩個都是空節點 → 相同 / Base 1: both null → same (nullptr 是 C++ 的空指標字面值)
        if (!p && !q) return true;   // !p 為真代表 p 是 nullptr / !p is true when p is nullptr

        // 邊界2：只有一個是空 → 結構不同 / Base 2: exactly one is null → different shape
        if (!p || !q) return false;

        // 邊界3：兩個都非空但值不同 → 不同 / Base 3: both exist but values differ → not same
        if (p->val != q->val) return false;

        // 遞迴比較左、右子樹，兩者皆真才回傳真 / Recurse on both subtrees; both must be true
        return isSameTree(p->left, q->left) && isSameTree(p->right, q->right);
    }
};