> For the complete documentation index, see [llms.txt](https://jimmylin1991.gitbook.io/practice-of-algorithm-problems/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://jimmylin1991.gitbook.io/practice-of-algorithm-problems/tree/298.-binary-tree-longest-consecutive-sequence.md).

# 298. Binary Tree Longest Consecutive Sequence

Given a binary tree, find the length of the longest consecutive sequence path.

The path refers to any sequence of nodes from some starting node to any node in the tree along the parent-child connections. The longest consecutive path need to be from parent to child (cannot be the reverse).

**Example 1:**

```
Input:

   1
    \
     3
    / \
   2   4
        \
         5

Output: 3

Explanation: Longest consecutive sequence path is 3-4-5, so return 3.
```

**Example 2:**

```
Input:

   2
    \
     3
    / 
   2    
  / 
 1

Output: 2 

Explanation: Longest consecutive sequence path is 2-3, not 3-2-1, so return 2.
```

{% hint style="info" %}
要找最長連續遞增的sequence，而且只能從parent往children的方向。
{% endhint %}

```cpp
void helper(TreeNode* node, int target, int curLen, int& res) { // time: O(n); space: O(n)
    if (!node) return;
    if (node->val == target) ++curLen;
    else curLen = 1;
    res = max(res, curLen);
    helper(node->left, node->val + 1, curLen, res);
    helper(node->right, node->val + 1, curLen, res);
}
int longestConsecutive(TreeNode* root) {
    if (!root) return 0;
    int res = 0;
    helper(root, root->val, 0, res);
    return res;
}
```

```cpp
void helper(TreeNode* node, int preVal, int curLen, int& res) { // time: O(n); space: O(n)
    if (!node) return;
    if ((preVal == -1) || (preVal != -1 && node->val == preVal + 1)) ++curLen;
    else curLen = 1;
    res = max(res, curLen);
    helper(node->left, node->val, curLen, res);
    helper(node->right, node->val, curLen, res);
}
int longestConsecutive(TreeNode* root) {
    if (!root) return 0;
    int res = 0;
    helper(root, -1, 0, res);
    return res;
}
```

{% content-ref url="/pages/-Lb4--M7bXWV1dwXVRZP" %}
[549. Binary Tree Longest Consecutive Sequence II](/practice-of-algorithm-problems/tree/549.-binary-tree-longest-consecutive-sequence-ii.md)
{% endcontent-ref %}