687. Longest Univalue Path

int longestUnivaluePath(TreeNode* root) {
if(!root) return 0;
int res=0;
helper(root,res);
return res;
}
int helper(TreeNode* node,int &res){
if(!node) return 0;
int left=helper(node->left,res);
int right=helper(node->right,res);
left=(node->left&&node->val==node->left->val)?left+1:0;
right=(node->right&&node->val==node->right->val)?right+1:0;
res=max(res,left+right);
return max(left,right);
}

int longestUnivaluePath(TreeNode* root) {
if(!root) return 0;
int sub=max(longestUnivaluePath(root->left),longestUnivaluePath(root->right));
return max(sub,helper(root->left,root->val)+helper(root->right,root->val));
}
int helper(TreeNode* node,int parent){
if(!node||node->val!=parent)  return 0;
return 1+max(helper(node->left,node->val),helper(node->right,node->val));
}