不得不說，Tree的題還算挺有意思的。首先給出two pass的做法： pass one找往下的sequence，存到map里；然后 pass two找往上的sequence，再更新全局變量.

class Solution {
public:

    int search_smaller(TreeNode *root){
        if(!root) return 0;
        int left = search_smaller(root->left);
        int right = search_smaller(root->right);
        int cur = 1;
        if(root->left && root->left->val == root->val - 1){
            cur = max(cur, left + 1); 
        }
        if(root->right && root->right->val == root->val - 1){
            cur = max(cur, right + 1);
        }
        mp[root] = cur;
        return cur;
    }
    
    void search_bigger(TreeNode *root){
        if(!root) return;
        max_count = max(max_count, mp[root]);
        if(root->left && root->left->val == root->val + 1){
            mp[root->left] = max(mp[root->left], mp[root] + 1);
        }
        if(root->right && root->right->val == root->val + 1){
            mp[root->right] = max(mp[root->right], mp[root] + 1);
        }
        search_bigger(root->left);
        search_bigger(root->right);
    }
    
    int longestConsecutive(TreeNode* root) {
        if(!root) return 0;
        search_smaller(root);
        search_bigger(root);
        return max_count;
    }
private:
    unordered_map<TreeNode*, int> mp;
    int max_count = 1;
};

網上果然有one pass的解法，就是把同一個節點的增序數目，和降序數目統計到一起，然后該節點序列為 增序 + 降序 - 1，即可

class Solution {
    
   struct ResultSet{
        int dec, inc;
        ResultSet(int d, int i) : dec(d), inc(i){}
   };
    
public:

    ResultSet search_util(TreeNode* root){
        if(!root) return ResultSet(0, 0);
        ResultSet left = search_util(root->left);
        ResultSet right = search_util(root->right);
        ResultSet ret(1, 1);
        if(root->left){
            if(root->left->val == root->val - 1){
                ret.inc = max(ret.inc, left.inc + 1);
            }else if(root->left->val == root->val + 1){
                ret.dec = max(ret.dec, left.dec + 1);
            }
        }
        if(root->right){
           if(root->right->val == root->val - 1){
                ret.inc = max(ret.inc, right.inc + 1);
            }else if(root->right->val == root->val + 1){
                ret.dec = max(ret.dec, right.dec + 1);
            } 
        }
        max_count = max(max_count, ret.inc + ret.dec - 1);
        return ret;
    }

    int longestConsecutive(TreeNode* root) {
        if(!root) return 0;
        search_util(root);
        return max_count;
    }
private:
    int max_count = 1;
};