实际上是找左右子树的最大高度

C++ 代码

/**
 * 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:
    int res;
    int diameterOfBinaryTree(TreeNode* root) {
        if(!root){
            return 0;
        }

         dfs( root);

        return res;
    }
    int dfs(TreeNode* root) {
        if(!root){
            return 0;
        }
        int left=dfs( root->left);
        int right=dfs( root->right);
        res=max(res,left+right);
        return max(left,right)+1;
    }
};