100. Same Tree

Recursion

/*
 * @lc app=leetcode id=100 lang=cpp
 *
 * [100] Same Tree
 *
 * https://leetcode.com/problems/same-tree/description/
 *
 * algorithms
 * Easy (63.69%)
 * Likes:    12070
 * Dislikes: 262
 * Total Accepted:    2.7M
 * Total Submissions: 4.2M
 * Testcase Example:  '[1,2,3]\n[1,2,3]'
 *
 * Given the roots of two binary trees p and q, 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: p = [1,2,3], q = [1,2,3]
 * Output: true
 *
 *
 * Example 2:
 *
 *
 * Input: p = [1,2], q = [1,null,2]
 * Output: false
 *
 *
 * Example 3:
 *
 *
 * Input: p = [1,2,1], q = [1,1,2]
 * Output: false
 *
 *
 *
 * Constraints:
 *
 *
 * The number of nodes in both trees is in the range [0, 100].
 * -10^4 <= Node.val <= 10^4
 *
 *
 */

// @lc code=start
/**
 * 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) {
        if (!p && !q) return true;
        if (!p || !q) return false;
        if (p->val != q->val) {
            return false;
        }
        return isSameTree(p->left, q->left) && isSameTree(p->right, q->right);
    }
};
// @lc code=end

• T: $O(N)$
• S: $O(N)$

Iteration

/**
 * 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)
    {
        queue<TreeNode*> q1, q2;
        q1.push(p); q2.push(q);
        while (!q1.empty()) {
            p = q1.front(); q1.pop();
            q = q2.front(); q2.pop();

            if (!p && !q) continue;

            if (!isSame(p, q)) return false;
            if (!isSame(p->left, q->left)) return false;
            if (!isSame(p->right, q->right)) return false;

            if (p->left)
            {
                q1.push(p->left);
                q2.push(q->left);
            }

            if (p->right)
            {
                q1.push(p->right);
                q2.push(q->right);
            }
        }
        return true;
    }
    bool isSame(TreeNode* p, TreeNode* q)
    {
        if (!p && !q) return true;
        if (!p || !q) return false;
        return p->val == q->val;
    }
};

• T: $O(N)$
• S: $O(N)$