100. Same Tree

• Recursion

/**
 * 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);
    }
};

• 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: $O(N)$