100.Same Tree

Tags: Tree Easy Depth-first Search

Links: https://leetcode.com/problems/same-tree/

---

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

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

Example 1:

Input:     1         1
          / \       / \
         2   3     2   3

        [1,2,3],   [1,2,3]

Output: true

Example 2:

Input:     1         1
          /           \
         2             2

        [1,2],     [1,null,2]

Output: false

Example 3:

Input:     1         1
          / \       / \
         2   1     1   2

        [1,2,1],   [1,1,2]

Output: false

---

/**
 * Definition for a binary tree node.
 * 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;
        s.push(p);
        s.push(q);

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

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

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

        return true;
    }
};

思路很直接，非递归形式使用辅助栈的前序遍历写法.

这道题作为easy类的题目，用递归写起来会代码长度小很多：

/**
 * Definition for a binary tree node.
 * 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 || !q) return !p && !q;
        return p->val == q->val && 
            isSameTree(p->left, q->left) &&
                isSameTree(p->right, q->right);
    }
};