101. Symmetric 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 isSymmetric(TreeNode* root) {
        return isSame(root, root);
    }
    bool isSame(TreeNode* p, TreeNode*  q) {
        if (!p && !q) return true;
        if (!p || !q) return false;
        if (p->val != q->val) return false;
        return isSame(p->left, q->right) && isSame(p->right, q->left);
    }
};

• T: $O(N)$
• S: $O(N)$

iteration

class Solution {
public:
    bool isSymmetric(TreeNode* root) {
        queue<TreeNode*> q;
        q.push(root);
        q.push(root);
        while (!q.empty()) {
            TreeNode* t1 = q.front(); q.pop();
            TreeNode* t2 = q.front(); q.pop();
            if (!t1 && !t2) continue;
            if (!t1 || !t2) return false;
            if (t1->val != t2->val) return false;
            q.push(t1->left);
            q.push(t2->right);
            q.push(t1->right);
            q.push(t2->left);
        }
        return true;
    }
};

• T: $O(N)$
• S: $O(N)$