Problem description:

For a binary tree T, we can define a flip operation as follows: choose any node, and swap the left and right child subtrees.

A binary tree X is flip equivalent to a binary tree Y if and only if we can make X equal to Y after some number of flip operations.

Write a function that determines whether two binary trees are flip equivalent. The trees are given by root nodes root1 and root2.

Example 1:

Input: root1 = [1,2,3,4,5,6,null,null,null,7,8], root2 = [1,3,2,null,6,4,5,null,null,null,null,8,7]
Output: true
Explanation: We flipped at nodes with values 1, 3, and 5.
Flipped Trees Diagram

Note:

Each tree will have at most 100 nodes.
Each value in each tree will be a unique integer in the range [0, 99].

Solution:

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/**
 * 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 flipEquiv(TreeNode* root1, TreeNode* root2) {
        if(!root1 && !root2) return true;
        if(!root1) return !root2;
        if(!root2) return !root1;
        if(root1->val != root2->val) return false;
        
        return (flipEquiv(root1->left, root2->left) && flipEquiv(root1->right, root2->right) || flipEquiv(root1->left, root2->right) && flipEquiv(root1->right, root2->left));
    }
};

time complexity: $O()$
space complexity: $O()$
reference: