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题目：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.
判断两棵树是否相同和之前的判断两棵树是否对称都是一样的原理，利用深度优先搜索DFS来递归。代码如下：
递归写法：
class Solution {
public:
bool isSameTree(TreeNode *p, TreeNode *q) {
if (!p && !q) return true;
if ((p && !q) || (!p && q) || (p->val != q->val)) return false;
return isSameTree(p->left, q->left) && isSameTree(p->right, q->right);
}
};
非递归写法：
class Solution {
public:
bool isSameTree(TreeNode* p, TreeNode* q) {
stack<TreeNode*> s1, s2;
if (p) s1.push(p);
if (q) s2.push(q);
while (!s1.empty() && !s2.empty()) {
TreeNode *t1 = s1.top(); s1.pop();
TreeNode *t2 = s2.top(); s2.pop();
if (t1->val != t2->val) return false;
if (t1->left) s1.push(t1->left);
if (t2->left) s2.push(t2->left);
if (s1.size() != s2.size()) return false;
if (t1->right) s1.push(t1->right);
if (t2->right) s2.push(t2->right);
if (s1.size() != s2.size()) return false;
}
return s1.size() == s2.size();
}
};