--- tags: leetcode --- # [543. Diameter of Binary Tree](https://leetcode.com/problems/diameter-of-binary-tree/) --- # My Solution ## The Key Idea for Solving This Coding Question ## C++ Code ```cpp= /** * 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: int diameterOfBinaryTree(TreeNode *root) { int diameter = 0; dfs(root, diameter); return diameter; } private: // dfs returns the length of the path. int dfs(TreeNode *root, int &diameter) { if (root == nullptr) { return 0; } int leftLen = dfs(root->left, diameter); int rightLen = dfs(root->right, diameter); int max1= max(leftLen, rightLen); int max2 = max(max1, leftLen + rightLen); diameter = max(max2, diameter); return max1 + 1; } }; ``` ## Time Complexity $O(n)$ $n$ is the number of nodes in the binary tree referred by `root`. ## Space Complexity $O(H)$ $H$ is the height of the binary tree referred by `root`. # Miscellane <!-- # Test Cases ``` [1,2,3,4,5] ``` ``` [1,2] ``` ``` [1,2,3] ``` ``` [-10,9,20,null,null,15,7] ``` ``` [-3] ``` ``` [-8,6,8,null,null,8,2,null,null,null,-2] ``` ``` [1] ``` ``` [5,1,4,null,null,3,6] ``` ``` [10,5,15,null,null,6,20] ``` ``` [1,null,1] ``` ``` [1,2,3,4,null,null,5] ``` ``` [3,9,20,null,null,15,7] ``` ``` [4,2,7,1,3] ``` ``` [18,2,22,null,null,null,63,null,84,null,null] ``` ``` [9,6,-3,null,null,-6,2,null,null,2,null,-6,-6,-6] ``` -->