687. Longest Univalue Path

    int len(TreeNode* node, int pa, int& mx) { // pa is the value of parent node
        if(!node) return 0;
        int l = len(node->left, node->val, mx);
        int r = len(node->right, node->val,mx);
        mx = max(mx, l+r);
        if(node->val == pa) return 1+max(l,r);
        else return 0;
    }
    int longestUnivaluePath(TreeNode* root) {
        if(!root) return 0;
        int mx = 0;
        // root doesn't have parent node, pick any value that is different
        len(root, root->val-1, mx);
        return mx;
    }