class Solution {
public:
    int ans=INT_MIN;
    int dfs(TreeNode*root){
        if(!root)return 0;
        int l=dfs(root->left);
        int r=dfs(root->right);
        ans=max(ans,l+r+1);
        return max(l,r)+1;
    }
    int diameterOfBinaryTree(TreeNode* root) {
        dfs(root);
        return ans-1;
    }
};