題目：

Given two binary trees, write a function to check if they are equal or not.

Two binary trees are considered equal if they are structurally identical and the nodes have the same value.

給定兩棵樹，寫一個函式判斷它們是否相等。

兩棵樹相等是指它們結構一致並且結點值相等。

程式碼遞迴版：

/**
 * 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) 
    {
        if(p == NULL && q == NULL)//遞迴結束條件
            return true;
        
        if(p == NULL || q == NULL)//剪枝
            return false;
            
        if(p->val == q->val)//當前結點值相等，繼續向下遍歷否則返回假
            return isSameTree(p->left,q->left) && isSameTree(p->right,q->right);
        else
            return false;
            
    }
};
 

/**
 * 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) 
    {
        stack<TreeNode*> s;//把q,q的結點成對入棧，再成對出棧進行比較
        s.push(p);
        s.push(q);
        while(!s.empty()) 
        {
        	p = s.top(); //取出棧頂
        	s.pop();     //刪除棧頂
        	q = s.top(); 
        	s.pop();
        	if (!p && !q) //p,q都為空，繼續遍歷，出棧判斷
        	    continue;
        	if (!p || !q) //p,q只有一個為空，則返回假 
        	    return false;
        	if (p->val != q->val) //p,q不等，返回假
        	    return false;
        	s.push(p->left);
        	s.push(q->left);
        	s.push(p->right);
        	s.push(q->right);
        }
        return true;
            
    }
};