Symmetric_Tree.cpp

// http://oj.leetcode.com/submissions/detail/3903517

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
 
// recursively, Time Complexity: O(n), Space Complexity: O(logn)
class Solution {
public:
    bool isSymmetric(TreeNode *root) {
        return root ? isSymmetric(root->left, root->right) : true;
    }
    bool isSymmetric(TreeNode* left, TreeNode* right)
    {
        if (!left && !right) return true; // leaf
        if (!left || !right) return false; // only has one branch
        return left->val == right->val
            && isSymmetric(left->left, right->right)
            && isSymmetric(left->right, right->left);
    }
};


// iteratively, Time Complexity: O(n), Space Complexity: O(logn)
class Solution {
public:
    bool isSymmetric(TreeNode *root) {
        if (!root) return true;
        stack<TreeNode*> s;
        s.push(root->left);
        s.push(root->right);
        
        while(!s.empty())
        {
            auto p = s.top(); s.pop();
            auto q = 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->right);
            s.push(p->right);
            s.push(q->left);
        }
        return true;
    }
};