[LeetCode 298] Binary Tree Longest Consecutive Sequence

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#LeetCode #递归 #二叉树

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本文介绍了一种在二叉树中寻找最长连续序列路径的方法。通过简单的递归算法，文章详细解释了如何从根节点开始，沿着父-子连接，找到最长的连续节点序列，并返回其长度。示例展示了输入树结构和预期输出，帮助理解算法的工作原理。

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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).

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Problem Correction

Example

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.

分析

简单的递归就可以计算。

Code

/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */

class Solution {
public:
    /**
     * @param root: the root of binary tree
     * @return: the length of the longest consecutive sequence path
     */
    int longestConsecutive(TreeNode * root) {
        // write your code here
        if (!root)
            return 0;
        
        int maxLen = 0;
        findLongest(root, 1, maxLen);
        return maxLen;
    }
    
    void findLongest(TreeNode* root, int len, int& maxLen)
    {
        if (!root)
            return;
        
        maxLen = max(maxLen, len);
        if (root->left)
        {
            if (root->val == root->left->val - 1)
                findLongest(root->left, len+1, maxLen);
            else
                findLongest(root->left, 1, maxLen);
        }
        
        if (root->right)
        {
            if (root->val == root->right->val - 1)
                findLongest(root->right, len+1, maxLen);
            else
                findLongest(root->right, 1, maxLen);
        }
    }
};

运行效率