---
tags: leetcode
---

# [951. Flip Equivalent Binary Trees](https://leetcode.com/problems/flip-equivalent-binary-trees/)
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# My Solution
## The Key Idea for Solving This Coding Question
DFS (recursion) preorder traversal.
If both subtrees are not empty, visit the subtree whose root value is smaller.
## C++ Code

/**
 * 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 flipEquiv(TreeNode *root1, TreeNode *root2) {
        vector<int> traversal1, traversal2;

        dfs(root1, traversal1);
        dfs(root2, traversal2);

        return traversal1 == traversal2;
    }

private:
    void dfs(TreeNode *root, vector<int> &traversal) {
        if (root == nullptr) {
            traversal.push_back(-1);
            return;
        }

        traversal.push_back(root->val);
        if (root->left == nullptr) {
            dfs(root->right, traversal);
            traversal.push_back(-1);
            return;
        }

        if (root->right == nullptr) {
            dfs(root->left, traversal);
            traversal.push_back(-1);
            return;
        }

        if (root->left->val < root->right->val) {
            dfs(root->left, traversal);
            dfs(root->right, traversal);
            return;
        }

        dfs(root->right, traversal);
        dfs(root->left, traversal);
    }
};

## Time Complexity
$O(n)$
$n$ is the number of nodes in the binary tree.
## Space Complexity
$O(H)$
$H$ is the height of the binary tree.
# Miscellaneous
<!--
# Test Cases

[1,2,3,4,5,6,null,null,null,7,8]
[1,3,2,null,6,4,5,null,null,null,null,8,7]

[]
[]

[]
[1]

[1,2,3]
[1,2,null,3]

[0,null,1]
[]

[0,null,1]
[0,1]

[1,2,3,4,5,6,null,null,null,7,8]
[99,3,2,null,6,4,5,null,null,null,null,8,7]

-->