543. Diameter of Binary Tree.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 helper(TreeNode *root, int &d)
    {
        if( root == NULL)
            return 0;
        int lh = helper(root->left,d);
        int rh = helper(root->right,d);
        d = max( lh+ rh, d);
        return 1 + max(lh,rh);
    }
    int diameterOfBinaryTree(TreeNode* root) {
        int d =0;
        helper(root,d);
        return d;
        
    }
};