leetcode100. Same Tree

easy程度题

Q:

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.

判断两个二叉树是否相同

思路：
类似于二叉树的前序遍历，将两个二叉树按照相同的顺序交替压栈，然后出栈比较栈顶的两个元素(两个二叉树位于相同位置的节点)是否相同

AC解：

class Solution {
public:
    bool isSameTree(TreeNode* p, TreeNode* q) 
    {//类似先序遍历
        stack<TreeNode*> s;
        s.push(q);
        s.push(p);

        while (!s.empty())
        {
            p = s.top();    s.pop();
            q = s.top();    s.pop();

            if (p == nullptr && q == nullptr)    continue;
            if (p == nullptr || q == nullptr)    return false;
            if (p->val != q->val)                return false;

            s.push(q->right);
            s.push(p->right);
            s.push(q->left);
            s.push(p->left);
        }

        return true;
    }
};

递归解法：

class Solution {
public:
    bool isSameTree(TreeNode* p, TreeNode* q)
    {
        if (p == nullptr && q == nullptr) return true; //终止条件
        if (p == nullptr || q == nullptr) return false;  //剪枝
        return p->val == q->val && isSameTree(p->left,q->left) && isSameTree(p->right,q->right);  //三方合并
    }
}